JP7185448B2 - Porous hollow fiber membrane, manufacturing method thereof, and filtration method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a porous hollow fiber membrane, a method for producing the same, and a filtration method using the porous hollow fiber membrane.

BACKGROUND ART Membrane filtration methods using hollow fiber membranes are becoming popular for clarification operations of liquids to be treated, such as water treatment and sewage treatment. Thermally induced phase separation is known as a method for producing hollow fiber membranes used in membrane filtration.

A thermally induced phase separation method uses a thermoplastic resin and an organic liquid. The thermally induced phase separation method using a solvent that does not dissolve the thermoplastic resin at room temperature but dissolves it at high temperature, that is, a latent solvent (poor solvent) as the organic liquid, involves kneading the thermoplastic resin and the organic liquid at high temperature, In this method, a plastic resin is dissolved in an organic liquid, cooled to room temperature to induce phase separation, and the organic liquid is removed to produce a porous body. This method has the following advantages.
(a) It is possible to form a film even with a polymer such as polyethylene that does not have a suitable solvent that can be dissolved at room temperature.
(b) Since the resin is melted at a high temperature and then solidified by cooling to form a film, especially when the thermoplastic resin is a crystalline resin, crystallization is promoted during film formation, and a high-strength film can be easily obtained.

Due to the above advantages, the thermally induced phase separation method is often used as a method for producing porous membranes. However, certain types of crystalline resins tend to have a spherulite film structure, and although they have high strength, they have low elongation and are brittle, which poses a problem in practical durability. Conventionally, a technique of forming a film using a poor solvent for a thermoplastic resin selected from citric acid esters has been disclosed (see Patent Document 1).

JP 2011-168741 A

However, there is a problem that the film produced by the method described in Patent Document 1 also has a spherulite structure. The present invention has been made in view of the above circumstances, and provides a porous hollow fiber membrane having a three-dimensional network structure and excellent chemical resistance and mechanical strength, and a method for producing the same.

In a known technique, when producing a porous hollow fiber membrane, a poor solvent is used as a raw material containing a thermoplastic resin for thermally induced phase separation. By mixing at least one of these, it was found that a film with a three-dimensional network structure excellent in chemical resistance and mechanical strength can be produced, leading to the present invention.

In addition, the inventors use a film-forming raw material that contains an additive, a non-solvent and a poor solvent/good solvent in a thermoplastic resin, and the non-solvent, poor solvent/good solvent becomes a non-solvent after being mixed. , the membrane structure expresses a three-dimensional network structure, and it has been found that a membrane having good porosity, high chemical resistance, and high mechanical strength can be produced. Furthermore, the inventors have found that the membrane filtration operation can be performed with high efficiency by performing filtration using the membrane produced as described above.

That is, the present invention is as follows.
(1)
The mixed liquid containing the thermoplastic resin, at least the first organic liquid and the second organic liquid and which is a non-solvent for the thermoplastic resin, and the additive are mixed and melt-kneaded, whereby the kneading is performed. a step of producing a product ; a step of discharging a kneaded product of the thermoplastic resin, the mixed liquid, and the additive; and an extraction removal of extracting the mixed liquid and the additive from the discharged kneaded product. and
The first organic liquid is sebacate, citrate, acetylcitrate, adipate, trimellitate, oleate, palmitate, stearate, phosphate, and has 6 or more carbon atoms. at least one selected from 30 or less fatty acids and epoxidized vegetable oils;
The second organic liquid is different from the first organic liquid and is sebacate, citrate, acetylcitrate, adipate, trimellitate, oleate, palmitate, stearic acid. at least one selected from esters, phosphate esters, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils ;
The thermoplastic resin is a fluororesin,
The method for producing a porous hollow fiber membrane , wherein the additive is finely divided silica .
(2 )
The first organic liquid is a non-solvent that does not uniformly dissolve 1/4 the mass of the thermoplastic resin of the first organic liquid at the boiling point of the first organic liquid (1 ), the method for producing a porous hollow fiber membrane.
( 3 )
The second organic liquid is a solvent that uniformly dissolves a quarter of the mass of the thermoplastic resin of the second organic liquid at a boiling point of the second organic liquid or lower (1 ) or the method for producing a porous hollow fiber membrane according to (2) .
( 4 )
The porous porous material according to any one of (1) to ( 3 ), wherein the mixed liquid does not dissolve 1/4 mass of the thermoplastic resin in the mixed liquid at the boiling point of the mixed liquid. A method for producing a hollow fiber membrane.
( 5 )
The method for producing a porous hollow fiber membrane according to any one of (1) to ( 4 ), wherein the thermoplastic resin is polyvinylidene fluoride .

The filtration method of the present invention performs filtration using the porous hollow fiber membrane of the present invention.

According to the present invention, there is provided a porous hollow fiber membrane having a three-dimensional network structure, good pore opening, high chemical resistance, and high mechanical strength.

FIG. 2 is a schematic diagram showing the membrane structure of the porous hollow fiber membrane according to the present embodiment. 2 is a schematic diagram showing the membrane structure of the porous hollow fiber membrane of Comparative Example 1. FIG.

Embodiments of the present invention will be described in detail. In addition, this invention is not limited to the following embodiment.

<Porous hollow fiber membrane>
The porous hollow fiber membrane of the present invention is described below as one embodiment.
FIG. 1 schematically shows the outer surface of a porous hollow fiber membrane according to one embodiment. The internal structure including the outer surface of the porous hollow fiber membrane 10 shown in FIG. 1 is not a spherulite structure but a three-dimensional network structure. By adopting a three-dimensional network structure, the tensile elongation at break is increased, and the resistance to alkalis (such as aqueous sodium hydroxide solution) and the like, which are frequently used as cleaning agents for membranes, is enhanced.

The porous hollow fiber membrane 10 contains a thermoplastic resin, and examples of the thermoplastic resin include polyolefins, copolymers of olefins and halogenated olefins, halogenated polyolefins, and mixtures thereof. Such thermoplastic resins include, for example, polyethylene, polypropylene, polyvinyl alcohol, ethylene-vinyl alcohol copolymer, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride (which may contain domains of hexafluoropropylene), or Mixtures of these may be mentioned. These materials are excellent in handleability due to their thermoplasticity and toughness, and therefore are excellent as membrane materials. Among these, homopolymers and copolymers of vinylidene fluoride, ethylene, tetrafluoroethylene, and chlorotrifluoroethylene, or mixtures of the above homopolymers and copolymers are excellent in mechanical strength and chemical strength (chemical resistance) and can be molded. It is preferred because of its good properties. More specifically, fluorine resins such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer, and ethylene-chlorotrifluoroethylene copolymer can be mentioned.

The porous hollow fiber membrane 10 contains components (such as impurities) other than the thermoplastic resin. The porous hollow fiber membrane 10 can contain up to about 5% by mass of components other than the thermoplastic resin. For example, as will be described later, the porous hollow fiber membrane 10 may contain the first organic liquid (hereinafter also referred to as non-solvent) or the second organic liquid (hereinafter also referred to as good solvent or poor solvent) used during production. ), or both. These organic liquids can be detected by pyrolysis GC-MS (Gas Chromatography-Mass Spectrometry).

In this embodiment, the non-solvent is an organic liquid that does not uniformly dissolve 1/4 mass of the thermoplastic resin at its boiling point. That is, the non-solvent is an organic liquid that does not uniformly dissolve the thermoplastic resin at the boiling point of the organic liquid in a mixed liquid containing the thermoplastic resin and the organic liquid at a mass ratio of 20:80. As the non-solvent, an organic liquid having the above properties is selected according to the thermoplastic resin applied to the porous hollow fiber membrane 10 . A refractive index or the like can be used to determine the dissolved state. For example, when a thermoplastic resin and an organic liquid are put into a glass test tube, the same refractive index is obtained regardless of which part of the mixed liquid is measured. In the undissolved state, it separates into two layers, each exhibiting a different refractive index.

In this embodiment, the poor solvent is an organic liquid that does not uniformly dissolve ¼ mass of the thermoplastic resin at 25° C. but uniformly dissolves it at least at the boiling point. That is, the poor solvent does not uniformly dissolve the thermoplastic resin at 25 ° C. in a mixed liquid containing a thermoplastic resin and an organic liquid at a mass ratio of 20:80, and heats at 100 ° C. or higher and below the boiling point of the organic liquid. It is an organic liquid that uniformly dissolves plastic resin. As the poor solvent, an organic liquid having the properties described above is selected according to the thermoplastic resin applied to the porous hollow fiber membrane 10 .

In this embodiment, the good solvent is an organic liquid that uniformly dissolves the thermoplastic resin at 25°C. As the good solvent, an organic liquid having the properties described above is selected according to the thermoplastic resin applied to the porous hollow fiber membrane 10 .

The first organic liquid is, for example, sebacate, citrate, acetylcitrate, adipate, trimellitate, oleate, palmitate, stearate, phosphate, phosphite It is at least one selected from esters, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils.

The second organic liquid is different from the first organic liquid and is sebacate, citrate, acetylcitrate, adipate, trimellitate, oleate, palmitate, stearate, It is at least one selected from phosphates, phosphites, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils.

Examples of fatty acids having 6 to 30 carbon atoms include capric acid, lauric acid, and oleic acid. Epoxidized vegetable oils include epoxidized soybean oil and epoxidized linseed oil. The above solvents are compatible with additives and have low toxicity.

The mixed liquid of the first organic liquid and the second organic liquid is a mixture in which the ratio of the thermoplastic resin and the mixed liquid is 20:80, and even if the boiling point of the mixed liquid is raised to It preferably does not dissolve. That is, it is preferable that the mixed liquid of the first organic liquid and the second organic liquid as a whole be a non-solvent for the thermoplastic resin.

The porous hollow fiber membrane 10 may further contain additives as components other than the thermoplastic resin. Hydrophobic additives are preferably used because they have good compatibility with the thermoplastic resin and the solvent. Inorganic substances may be used as additives. Inorganic fine powder is preferred as the inorganic material. Specific examples of the inorganic fine powder include silica (including fine silica powder), titanium oxide, lithium chloride, calcium chloride, organic clay, etc. Among these, fine silica powder is preferable from the viewpoint of cost. When organic substances are used as additives, organic clays and the like are preferably used.

(Physical properties of porous hollow fiber membrane 10)
Next, physical properties of the porous hollow fiber membrane 10 according to this embodiment will be described.
The initial value of the tensile elongation at break of the porous hollow fiber membrane 10 is preferably 60% or more, more preferably 80% or more, still more preferably 100% or more, and particularly preferably 120% or more. The tensile elongation at break can be measured by the measuring method in Examples described later.

Alkali resistance can be evaluated by the ratio of elongation at break before and after immersion in alkali. For example, after immersing the porous hollow fiber membrane 10 in a 4% NaOH aqueous solution for 10 days, the tensile elongation at break ratio, which is the tensile rupture elongation after immersion to the initial tensile rupture elongation before immersion, is 60% or more. is preferred. The tensile elongation ratio at break is more preferably 65% or more, and still more preferably 70% or more.

From a practical point of view, the compressive strength of the porous hollow fiber membrane 10 is 0.2 MPa or more, preferably 0.3 to 1.0 MPa, more preferably 0.4 to 1.0 MPa. Compressive strength is measured by measuring the pure water permeation rate due to external pressure, increasing the pressure every 0.05 MPa, judging that the pressure at which the pressure and the pure water permeation rate are no longer proportional is that the membrane has collapsed, and compressing the pressure immediately before that. It can be measured as intensity.

The open area ratio (surface open area ratio) of the surface of the porous hollow fiber membrane 10 is 20 to 60%, preferably 25 to 50%, and more preferably 25 to 45%. By using a membrane with a surface opening ratio of 20% or more on the side that contacts the liquid to be treated for filtration, both deterioration of water permeability due to clogging and abrasion of the membrane surface are reduced, and filtration stability is improved. be able to. The aperture ratio can be measured by the measuring method in Examples described later.

Even if the opening ratio is high, if the pore size is too large, the required separation performance may not be exhibited. Therefore, the pore diameter on the outer surface is 1,000 nm or less, preferably 10 to 800 nm, more preferably 100 to 700 nm. If the pore diameter is 1,000 nm or less, it is possible to block the components contained in the liquid to be treated, and if it is 10 nm or more, sufficiently high water permeability can be ensured. The pore diameter can be measured by the measurement method in Examples described later.

The thickness of the porous hollow fiber membrane 10 is preferably 80-1,000 μm, more preferably 100-300 μm. When the thickness is 80 μm or more, the strength is increased, and when the thickness is 1,000 μm or less, pressure loss due to membrane resistance is reduced.

The porosity of the porous hollow fiber membrane 10 is preferably 50-80%, more preferably 55-65%. When the porosity is 50% or more, the water permeability is high. On the other hand, when the porosity is 80% or less, the mechanical strength can be increased. In addition, in this embodiment, the porosity is calculated by the following formula.
Porosity [%] = 100 × {(wet membrane weight [g]) - (dry membrane weight [g])} / (membrane volume [cm ³ ])

In the present embodiment, the wet membrane is a membrane in which the pores are filled with water but the hollow portions are not filled with water. Specifically, a sample membrane having a length of 10 to 20 cm is immersed in ethanol to fill the pores with ethanol, and then the immersion in water is repeated 4 to 5 times to sufficiently replace the pores with water. A wet membrane was obtained by holding one end of the substituted hollow fiber membrane by hand and shaking it well about 5 times, then holding it at the other end and shaking it about 5 times again to remove the water in the hollow portion. A dry film was obtained by drying in an oven at 80° C. to constant weight after weighing the wet film. The membrane volume was obtained by membrane volume [cm ³ ]=π×{(outer diameter [cm]/2) ² −(inner diameter [cm]/2) ² }×(membrane length [cm]).

As for the shape of the porous hollow fiber membrane 10, an annular single-layer membrane can be given, but a multi-layer membrane having different pore diameters between the separation layer and the support layer supporting the separation layer may also be used. Also, the outer surface and the inner surface may have a modified cross-sectional structure such as having projections.

(Liquid to be processed)
Liquids to be treated by the porous hollow fiber membrane 10 are, for example, suspended water and process liquids. The porous hollow fiber membrane 10 is suitably used for a water purification method including a step of filtering suspended water.

Suspended water includes natural water, domestic wastewater, and treated water thereof. Examples of natural water include river water, lake water, ground water, and sea water. Suspended water to be treated also includes treated natural water obtained by subjecting natural water to treatments such as sedimentation treatment, sand filtration treatment, coagulating sedimentation sand filtration treatment, ozone treatment, and activated carbon treatment. An example of domestic wastewater is sewage. Primary treated sewage water subjected to screen filtration or sedimentation treatment, secondary treated sewage water subjected to biological treatment, further treated with coagulation sedimentation sand filtration, activated carbon treatment, ozone treatment, etc.3 Secondary treated (advanced treated) water is also included in the suspended water to be treated. These suspended waters contain turbidity (humus colloid, organic colloid, clay, bacteria, etc.) composed of fine organic matter, inorganic matter, and organic-inorganic mixtures of μm order or less.

Generally, the water quality of suspended water such as natural water, domestic wastewater, and treated water thereof can be expressed by turbidity and organic matter concentration, which are typical water quality indicators, alone or in combination. When water quality is classified by turbidity (average turbidity, not instantaneous turbidity), it can be broadly classified into low turbid water with a turbidity of less than 1, medium turbid water with a turbidity of 1 or more and less than 10, high turbid water with a turbidity of 10 or more and less than 50, It can be classified as ultra-high turbid water with a turbidity of 50 or higher. In addition, if the water quality is classified by the concentration of organic matter (Total Organic Carbon (TOC): mg / L) (this is also an average value instead of an instantaneous value), it can be roughly classified into low TOC water of less than 1, 1 It can be classified into medium TOC water of 4 or more and less than 4, high TOC water of 4 or more and less than 8, super high TOC water of 8 or more, and the like. Basically, the higher the turbidity or TOC of water, the more likely it is to clog the filtration membrane.

The process liquid refers to a liquid to be separated when valuables and non-valuables are separated in food, pharmaceutical, semiconductor manufacturing, and the like. In food production, the porous hollow fiber membrane 10 is used, for example, when separating alcoholic beverages such as Japanese sake and wine from yeast. In the manufacture of pharmaceuticals, the porous hollow fiber membrane 10 is used, for example, for sterilization during protein purification. In semiconductor manufacturing, the porous hollow fiber membrane 10 is used, for example, for separating abrasives and water from polishing wastewater.

<Method for producing porous hollow fiber membrane 10>
Next, a method for producing the porous hollow fiber membrane 10 will be described. The method for producing the porous hollow fiber membrane 10 includes (a) a step of preparing a melt-kneaded product, and (b) supplying the melt-kneaded product to a multi-structure spinning nozzle and extruding the melt-kneaded product from the spinning nozzle to form a hollow hollow fiber. (c) extracting an organic liquid from the hollow fiber membrane; and (d) extracting an additive from the hollow fiber membrane.

In steps (c) and (d), the non-solvent and inorganic fine powder are extracted from the hollow fiber membrane, and the hollow fiber membrane after extraction contains the non-solvent and inorganic fine powder as residuals.

As the extractant used in step (c), it is preferable to use liquids such as methylene chloride and various alcohols that do not dissolve thermoplastic resins but have high affinity with plasticizers.

As the extractant in step (d), it is preferable to use hot water or a liquid that can dissolve the additives used, such as acids and alkalis, but does not dissolve the thermoplastic resin.

Next, details of the step (a) of preparing the melt-kneaded product in the method for producing the porous hollow fiber membrane 10 of the present embodiment will be described.

The step (a) includes a step of absorbing the first organic liquid and the second organic liquid into an additive to form a powder, and a step of mixing the powder and the thermoplastic resin. In the step of mixing the powder and the thermoplastic resin, a melt-kneaded product is prepared by using an extruder with the powder composed of the three components of the organic liquid, the additive, and the thermoplastic resin. As will be described later, the first organic liquid contains at least a non-solvent, and the mixed liquid after being mixed with the second organic liquid is also a non-solvent for the thermoplastic resin used.

In this embodiment, as the thermoplastic resin used in step (a), as described above, for example, a polyolefin, a copolymer of an olefin and a halogenated olefin, a halogenated polyolefin, or a mixture thereof is used. . More specifically, examples of thermoplastic resins include polyethylene, polypropylene, polyvinyl alcohol, ethylene-vinyl alcohol copolymer, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride (including domains of hexafluoropropylene), may be used), or a mixture thereof. These materials are excellent in handleability due to their thermoplasticity and toughness, and therefore are excellent as membrane materials. Among these, homopolymers and copolymers of vinylidene fluoride, ethylene, tetrafluoroethylene, and chlorotrifluoroethylene, or mixtures of the above homopolymers and copolymers are excellent in mechanical strength and chemical strength (chemical resistance) and can be molded. It is preferred because of its good properties. More specifically, fluorine resins such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer, and ethylene-chlorotrifluoroethylene copolymer are preferred.

The concentration of the thermoplastic resin in the melt-kneaded product is preferably 20-60% by mass, more preferably 25-45% by mass, still more preferably 30-45% by mass. When this value is 20% by mass or more, the mechanical strength can be increased, and on the other hand, when it is 60% by mass or less, the water permeability can be increased.

In this embodiment, as the first organic liquid used in step (a), it is preferable to use a non-solvent for the thermoplastic resin. Also, as the second organic liquid used in step (a), it is preferable to use a good solvent or poor solvent for the thermoplastic resin. In step (a), a non-solvent for the thermoplastic resin is mixed with a good solvent or poor solvent. By using a non-solvent as the raw material of the membrane in this way, the porous hollow fiber membrane 10 having a three-dimensional network structure can be obtained. Its mechanism of action is not necessarily clear, but it is thought that the crystallization of the polymer is moderately inhibited by using a solvent with a lower solubility by mixing it with a non-solvent, making it easier to form a three-dimensional network structure. . The concentration of the first organic liquid in the melt-kneaded product is preferably 10-60% by mass. The concentration of the second organic liquid in the melt-kneaded product is preferably 20-50% by mass.

As described above, the non-solvent, the poor solvent or the good solvent are, for example, sebacate ester, citrate ester, acetyl citrate ester, adipate ester, It is selected from various esters such as trimellitate, oleate, palmitate, stearate, phosphate, phosphite, fatty acid having 6 to 30 carbon atoms, and epoxidized vegetable oil.

An organic liquid capable of dissolving a thermoplastic resin in the temperature range of 25°C to 100°C is a good solvent, an organic liquid capable of dissolving the thermoplastic resin in a temperature range of 100°C to the boiling point is a poor solvent for the thermoplastic resin, An organic liquid that cannot be dissolved even at a boiling point or higher is called a non-solvent. In this embodiment, a good solvent, a poor solvent, and a non-solvent can be determined as follows.

Good solvents that can be applied as the second organic liquid include sebacate, citrate, acetylcitrate, adipate, trimellitate, oleate, palmitate, stearate, and phosphate. , a phosphite ester, a fatty acid having 6 to 30 carbon atoms, and at least one ester selected from various esters such as epoxidized vegetable oils, in a mixed solution in which the mass is four times that of the thermoplastic resin, It is an organic liquid that uniformly dissolves the thermoplastic resin in the solvent at any temperature within the range of higher than 25° C. and not higher than the boiling point of the mixed liquid.

Poor solvents that can be applied as the second organic liquid include sebacate, citrate, acetylcitrate, adipate, trimellitate, oleate, palmitate, stearate, and phosphate. , a phosphite ester, a fatty acid having 6 to 30 carbon atoms, and at least one ester selected from various esters such as epoxidized vegetable oils, in a mixed solution in which the mass is four times that of the thermoplastic resin, An organic liquid that does not uniformly dissolve the thermoplastic resin when the temperature of the mixed liquid is 25 ° C., and uniformly dissolves the thermoplastic resin at any temperature in the range of higher than 100 ° C. and below the boiling point. be.

Non-solvents applied as the first organic liquid include sebacate, citrate, acetylcitrate, adipate, trimellitate, oleate, palmitate, stearate, phosphate , at least one selected from fatty acids having 6 to 30 carbon atoms, and various esters such as epoxidized vegetable oils. It is an organic liquid in which the thermoplastic resin does not uniformly dissolve the non-solvent even if the solvent is raised to.

Specifically, about 2 g of thermoplastic resin and about 8 g of organic liquid are put in a test tube, and the test tube block heater is used to determine whether the solvent is a good solvent, a poor solvent, or a non-solvent. Heat up to the boiling point of the organic liquid, mix in the test tube with a spatula or the like, and judge the solubility in the above temperature range. In other words, a good solvent or a poor solvent is one in which the thermoplastic resin dissolves, and a non-solvent is one in which the thermoplastic resin does not dissolve.

The boiling points of some specific examples of the above esters listed as the first organic liquid and the second organic liquid are as follows. The boiling point of tributyl acetylcitrate is 343°C, dibutyl sebacate is 345°C, dibutyl adipate is 305°C, diisobutyl adipate is 293°C, and bis-2-ethylhexyl adipate is 335°C. , diisononyl adipate is above 250°C, diethyl adipate is 251°C, triethyl citrate is 294°C, and triphenylphosphite is 360°C.

For example, when polyvinylidene fluoride (PVDF) is used as the thermoplastic resin and acetyl tributyl citrate, dibutyl sebacate, or dibutyl adipate is used as the organic liquid, PVDF does not uniformly dissolve in these solvents at 25°C. When the temperature of the mixture is raised, PVDF is uniformly mixed and dissolved in these solvents at any temperature above 100° C. and below the boiling point. Therefore, acetyl tributyl citrate, dibutyl sebacate, and dibutyl adipate are poor solvents for PVDF. On the other hand, when bis-2-ethylhexyl adipate, diisononyl adipate, bis-2-ethylhexyl sebacate, or oleic acid is used as the organic liquid, PVDF does not dissolve even at their boiling points. Therefore, bis-2-ethylhexyl adipate, diisononyl adipate, and bis-2-ethylhexyl sebacate and oleic acid are non-solvents for PVDF.

Also, when ethylene-tetrafluoroethylene copolymer (ETFE) is used as the thermoplastic resin and diethyl adipate is used as the organic liquid to be mixed, ETFE does not dissolve uniformly at 25°C, and when the temperature is higher than 100°C and below the boiling point, Any temperature within the range will mix and melt uniformly. Therefore, diethyl adipate is a poor solvent for ETFE. On the other hand, ETFE does not dissolve when bis-2-ethylhexyl adipate, diisononyl adipate, or capric acid is used as the organic liquid. Thus, bis-2-ethylhexyl adipate, diisononyl adipate, and capric acid are non-solvents for ETFE.

When ethylene-monochlorotrifluoroethylene copolymer (ECTFE) is used as the thermoplastic resin and triethyl citrate or bis-2-ethylhexyl adipate is used as the organic liquid to be mixed, ECTFE dissolves uniformly at 25°C. ECTFE dissolves uniformly in the solvent at any temperature within the range above 100° C. and below the boiling point. Therefore, triethyl citrate and bis-2-ethylhexyl adipate are poor solvents for ECTFE. On the other hand, ECTFE does not dissolve when triphenylphosphite or oleic acid is used as the organic liquid. Therefore, triphenylphosphite and oleic acid are non-solvents for ECTFE.

In addition, when polyethylene (PE) is used as the thermoplastic resin and dibutyl sebacate is used as the organic liquid to be mixed, PE does not dissolve uniformly at 25°C, and at any temperature higher than 100°C and not higher than the boiling point. PE is uniformly mixed and dissolved. Therefore, for PE, dibutyl sebacate is a poor solvent. On the other hand, when bis-2-ethylhexyl adipate or acetyl tributyl citrate is used as the organic liquid, PE does not dissolve. Thus, bis-2-ethylhexyl adipate and acetyl tributyl citrate are non-solvents for PE.

In this embodiment, the additive used in step (a) may be inorganic or organic. When an inorganic substance is used as an additive, the inorganic substance is preferably inorganic fine powder. The primary particle size of the inorganic fine powder contained in the melt-kneaded product is preferably 50 nm or less, more preferably 5 nm or more and less than 30 nm. Specific examples of the inorganic fine powder include silica (including fine silica powder), titanium oxide, lithium chloride, calcium chloride, organic clay, etc. Among these, fine silica powder is preferable from the viewpoint of cost. The above-mentioned "primary particle size of inorganic fine powder" means a value obtained from analysis of electron micrographs. That is, first, a group of inorganic fine powders is pretreated by the method of ASTM D3849. After that, the diameters of 3000 to 5000 particles photographed on the transmission electron micrograph are measured, and these values are arithmetically averaged to calculate the primary particle diameter of the inorganic fine powder. The material present in the inorganic fine powder in the porous hollow fiber membrane 10 can be determined by identifying the existing element using fluorescent X-rays or the like. When organic substances are used as additives, organic clays and the like are preferably used.

<Filtration method>
The filtration method of this embodiment uses the porous hollow fiber membrane 10 of this embodiment to filter the liquid to be treated. Filtration can be performed with high efficiency by using the porous hollow fiber membrane 10 of the present embodiment.

EXAMPLES The present invention will be described in detail below with reference to Examples, but the present invention is not limited to these. Each physical property value in Examples and Comparative Examples was obtained by the following methods.

(1) Membrane outer diameter and inner diameter The hollow fiber membrane was thinly sliced with a razor, and the outer diameter and inner diameter were measured with a 100-fold magnifying glass. For one sample, measurements were made at 60 points at intervals of 30 mm.

(2) Observation of open area ratio, pore diameter, and membrane structure Using an electron microscope SU8000 series manufactured by HITACHI, electron microscope (SEM) images of the surface and cross section of the membrane were taken at a magnification of 5000 at an acceleration voltage of 3 kV. Cross-sectional electron microscopy samples were obtained by slicing membrane samples frozen in ethanol. Next, using the image analysis software Winroof 6.1.3, "noise removal" of the SEM image is performed with a numerical value of "6", and further binarization with a single threshold is performed, and a binary value is obtained with "threshold: 105". valuated. By calculating the occupied area of the pores in the binarized image thus obtained, the aperture ratio of the membrane surface was obtained.

The pore diameter is determined by adding the pore area of each pore in order from the smallest pore diameter to each pore existing on the surface, and the sum of the pore diameters reaches 50% of the total pore area of each pore. did.

For the film structure, the appearance of the film surface and the cross section photographed at a magnification of 5,000 was observed, and a three-dimensional network structure was determined to be one in which there was no spherulite and the polymer trunk exhibited a three-dimensional network structure.

(3) Water Permeability After immersing in ethanol and then repeatedly immersing in pure water several times, one end of a wet hollow fiber membrane with a length of about 10 cm is sealed, an injection needle is inserted into the hollow part of the other end, and the membrane is placed in an environment of 25°C. Inject pure water at 25 ° C from the injection needle into the hollow part at a pressure of 0.1 MPa, measure the amount of pure water permeating from the outer surface, determine the pure water flux by the following formula, and evaluate water permeability did.
Pure water flux [L/m ² /h] = 60 × (permeated water amount [L]) / {π × (membrane outer diameter [m]) × (membrane effective length [m]) × (measurement time [min]) }

Here, the effective membrane length refers to the net membrane length excluding the portion where the injection needle is inserted.

(4) Tensile elongation at break (%)
The load and displacement at tensile breakage were measured under the following conditions.
According to the method of JIS K7161, hollow fiber membranes were used as they were as samples.
Measuring equipment: Instron type tensile tester (AGS-5D manufactured by Shimadzu Corporation)
Distance between chucks: 5cm
Tensile speed: 20 cm/min From the obtained results, the tensile elongation at break was calculated according to JIS K7161.

(5) Water Permeability Retention Rate During Filtration of Suspended Water The water permeability retention rate during filtration of suspended water is an index for judging the degree of deterioration of water permeability performance due to clogging (fouling). For the measurement, the wet hollow fiber membrane which had been immersed in ethanol and then immersed in pure water several times was filtered at an effective membrane length of 11 cm by an external pressure method. First, pure water was filtered at a filtration pressure of 10 ^{m 3} per day per 1 m 2 of the outer surface of the membrane, and the permeated water was collected for 2 minutes to obtain the initial pure water permeability. Next, river surface water (Fuji River surface water: turbidity 2.2, TOC concentration 0.8 ppm), which is natural suspended water, was filtered for 10 minutes at the same filtration pressure as when measuring the initial pure water permeability. , the permeated water was sampled for 2 minutes from the 8th minute to the 10th minute of filtration, and used as the water permeation amount at the time of filtration of the suspended water. The water permeability retention rate during filtration of suspended water was defined by the following formula. All operations were carried out at 25° C. and a film surface linear velocity of 0.5 m/sec.
Permeability retention rate [%] during suspension water filtration = 100 × (permeation amount during suspension water filtration [g]) / (initial pure water permeation amount [g])
Each parameter in the formula is calculated by the following formula.
Filtration pressure = {(input pressure) + (output pressure)}/2
Membrane outer surface area [m ² ] = π x (yarn outer diameter [m]) x (membrane effective length [m])
Membrane surface linear velocity [m/s] = 4 × (circulating water volume [m ³ /s]) / {π × (tube diameter [m]) ² - π × (membrane outer diameter [m]) ² }

In this measurement, the filtration pressure of suspended water is not the same for each membrane, but the initial pure water permeability (also the permeability at the start of suspension filtration) is the filtration pressure that permeates 10 m ³ per day per 1 m ² of the outer surface of the membrane. set. This is because in actual water treatment and sewage treatment, membranes are usually used in a quantitative filtration operation (a method of filtering by adjusting the filtration pressure so that a certain amount of filtered water can be obtained within a certain period of time). Therefore, in this measurement, it is possible to compare the water permeability deterioration under conditions as close as possible to the conditions of the quantitative filtration operation within the range of measurement using one hollow fiber membrane.

(6) Chemical resistance test A porous hollow fiber membrane that has been wetted with 100% ethanol and then replaced with pure water was cut into 10 cm pieces, and 20 pieces were immersed in 500 ml of a 4% sodium hydroxide aqueous solution. It was kept at 40°C for days. The tensile elongation at break of the film before and after immersion in sodium hydroxide was measured with n20, and the average value was calculated. Elongation retention rate was defined as 100×(elongation after immersion)/(elongation before immersion) to evaluate chemical resistance.

[Example 1]
The melt-kneaded product was extruded using a spinning nozzle having a double tube structure to obtain a porous hollow fiber membrane of Example 1. PVDF resin (manufactured by Solvay Specialty Polymers Co., Ltd., Solef 6010) 40% by mass as a thermoplastic resin, fine powder silica (R972 manufactured by Nippon Aerosil Co., Ltd.) 23% by mass as an additive, bis 2-ethylhexyl adipate (Tokyo A melt-kneaded product was prepared using 32% by mass of DOA manufactured by Kasei Kogyo Co., Ltd., boiling point 335° C.) and 5% by mass of acetyl tributyl citrate (ATBC manufactured by Tokyo Chemical Industry Co., Ltd., boiling point 343° C.) as a second organic liquid. The temperature of the melt-kneaded product was about 200°C to 250°C.

The extruded hollow fiber-like molding was passed through a free running distance of 120 mm, and then solidified in water at 30° C. to prepare a porous hollow fiber membrane by a melt film-forming method. It was taken up at a speed of 5 m/min and wound into a skein. The resulting hollow fiber extrudate was immersed in isopropyl alcohol to extract and remove bis-2-ethylhexyl adipate and acetyl tributyl citrate. Subsequently, the hollow fiber membrane was immersed in water for 30 minutes to replace the water. Subsequently, it was immersed in a 20% by mass NaOH aqueous solution at 70° C. for 1 hour, and then repeatedly washed with water to extract and remove fine silica powder.

Table 1 shows the compounding composition, manufacturing conditions, and various performances of the obtained porous hollow fiber membrane of Example 1. The membrane structure of the porous hollow fiber membrane of Example 1 exhibited a three-dimensional network structure as shown in FIG.

[Example 2]
A melt-kneaded product was prepared by using 5% by mass of dibutyl sebacate (DBS manufactured by Tokyo Chemical Industry Co., Ltd., boiling point of 345°C) instead of 5% by mass of acetyl tributyl citrate (ATBC, boiling point of 343°C) as the second organic liquid. A porous hollow fiber membrane was produced in the same manner as in Example 1 except for the above.

Table 1 shows the compounding composition, production conditions, and various performances of the porous hollow fiber membrane obtained in Example 2. The membrane structure of the porous hollow fiber membrane of Example 2 exhibited a three-dimensional network structure as shown in FIG.

[Example 3]
Melt-kneaded product using 32% by mass of diisononyl adipate (DINA manufactured by Tokyo Chemical Industry Co., Ltd., boiling point of 250°C or higher) instead of 32% by mass of bis-2-ethylhexyl adipate (DOA, boiling point of 335°C) as the first organic liquid A porous hollow fiber membrane was produced in the same manner as in Example 1, except that the was prepared.

Table 1 shows the compounding composition, manufacturing conditions, and various performances of the porous hollow fiber membrane obtained in Example 3. The membrane structure of the porous hollow fiber membrane of Example 3 exhibited a three-dimensional network structure as shown in FIG.

[Comparative Example 1]
A hollow fiber membrane of Comparative Example 1 was obtained in the same manner as in Example 1, except that the melt-kneaded product was prepared without mixing the first organic liquid. Table 1 shows the compounding composition, production conditions, and various performances of the obtained porous hollow fiber membrane of Comparative Example 1. The membrane structure of the porous hollow fiber membrane of Comparative Example 1 exhibited a spherulite structure as shown in FIG.

As shown in Tables 1 and 2, in Examples 1 to 3, good porosity, chemical resistance, and mechanical properties were obtained by mixing a non-solvent with the film-forming stock solution in film formation by the melt film-forming method. It can be seen that a porous hollow fiber membrane with high strength is produced.
On the other hand, Comparative Example 1, which does not contain a non-solvent, has a spherulitic pore structure and is inferior in openness, chemical resistance, and mechanical strength.

According to the present invention, since the porous hollow fiber membrane is formed containing a non-solvent, a porous hollow fiber membrane having good pore opening, high chemical resistance, and high mechanical strength is provided.

10 Porous hollow fiber membrane

Claims (5)

The mixed liquid containing the thermoplastic resin, at least the first organic liquid and the second organic liquid and which is a non-solvent for the thermoplastic resin, and the additive are mixed and melt-kneaded, whereby the kneading is performed. a step of producing a product ; a step of discharging a kneaded product of the thermoplastic resin, the mixed liquid, and the additive; and an extraction removal of extracting the mixed liquid and the additive from the discharged kneaded product. and
The first organic liquid is sebacate, citrate, acetylcitrate, adipate, trimellitate, oleate, palmitate, stearate, phosphate, and has 6 or more carbon atoms. at least one selected from 30 or less fatty acids and epoxidized vegetable oils;
The second organic liquid is different from the first organic liquid and is sebacate, citrate, acetylcitrate, adipate, trimellitate, oleate, palmitate, stearic acid. at least one selected from esters, phosphate esters, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils ;
The thermoplastic resin is a fluororesin,
The method for producing a porous hollow fiber membrane , wherein the additive is finely divided silica . 3. The first organic liquid is a non-solvent that does not uniformly dissolve 1/4 mass of the thermoplastic resin of the first organic liquid at the boiling point of the first organic liquid. 2. The method for producing a porous hollow fiber membrane according to 1 . 3. The second organic liquid is a solvent that uniformly dissolves a quarter of the mass of the thermoplastic resin of the second organic liquid below the boiling point of the second organic liquid. 3. The method for producing a porous hollow fiber membrane according to 1 or 2 . 4. The porous hollow fiber according to any one of claims 1 to 3 , wherein the mixed liquid does not dissolve 1/4 mass of the thermoplastic resin in the mixed liquid at the boiling point of the mixed liquid. Membrane manufacturing method. The method for producing a porous hollow fiber membrane according to any one of claims 1 to 4 , wherein the thermoplastic resin is polyvinylidene fluoride.

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