JP2006231274A - Hollow fiber membrane, hollow fiber membrane module using it, membrane filtering device and water treating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hollow fiber membrane for treating water having a high stain resistance which uses a poly (vinylidene fluoride) based resin with excellent chemical endurance life and physical strength. <P>SOLUTION: In the hollow fiber membrane comprising the poly(vinylidene fluoride) based resin, the average diameter of the pore in the surface of water to be treated side of the hollow fiber membrane is 0.01-0.1 μm, the pore density in the surface of the water to be treated side is 1×10<SP>13</SP>-1×10<SP>5</SP>pieces/m<SP>2</SP>and the opening ratio in the surface of the water to be treated side is 3-30%. The hollow fiber membrane consists of the poly(vinylidene fluoride) based resin is disclosed in which the average diameter of the pore in the surface of water to be treated side of the hollow fiber membrane is 0.01-0.1 μm, the pore density in the surface of the water to be treated side is 1×10<SP>13</SP>-1×10<SP>5</SP>pieces/m<SP>2</SP>and the opening ration in the surface of the water to be treated side is 3-30%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  Hollow fiber microfiltration membranes, hollow fiber ultrafiltration membranes used in water treatment such as water purification, industrial water production, wastewater treatment, reverse osmosis membrane pretreatment, and hollow fiber membrane modules, membrane filtration devices, permeation using the same The present invention relates to a method for producing water.

  Separation membranes such as microfiltration membranes and ultrafiltration membranes are used in various fields such as the food industry, medical field, water production, and wastewater treatment field. Particularly in recent years, separation membranes are increasingly used in the field of drinking water production, that is, in the process of water purification.

  Especially for separation membranes used in the field of drinking water production, oxidizing agents such as sodium hypochlorite are added to treated water and backwash water for the purpose of sterilizing permeated water and reducing membrane fouling. It is essential to clean the membrane with acids such as hydrochloric acid, citric acid and oxalic acid, alkalis such as aqueous sodium hydroxide, chlorine, surfactants, etc., and high chemical durability against these chemicals is required. It has been. In addition, since the end of the 20th century, the problem that chlorine-resistant pathogenic microorganisms such as Cryptosporidium are mixed in drinking water has become obvious, so that the water to be treated is not mixed into the permeated water because the membrane is damaged. High physical strength and physical durability are also required. Against this background, in recent years, a separation membrane using a polyvinylidene fluoride resin as a material having high chemical durability, high physical strength, and physical durability has attracted attention, and its application tends to expand in the future.

  When used in water treatment applications such as water purification, a hollow fiber membrane having a large effective membrane area per unit volume is generally employed because of the large amount of water that must be treated. Furthermore, if the water permeability of the hollow fiber membrane is excellent, the required membrane area will be reduced and the equipment will be compact, so that the equipment cost can be saved, and it will be advantageous in terms of membrane replacement costs and plant installation site area. .

  However, the water permeation performance decreases with the continuation of the filtration operation due to contamination of the membrane by various components contained in the water to be treated. In particular, separation membranes using polyvinylidene fluoride resins have excellent chemical durability and physical strength characteristics, but they are hydrophobic and are easily soiled. A device for the stain resistance of a hollow fiber membrane made of a plastic resin has been applied.

  The problem of membrane fouling, or “fouling”, is an extremely complex phenomenon, and is a common issue for separation membranes in the field of water treatment. Many researchers and engineers have clarified fouling mechanisms and controlled fouling. However, it is far from being able to establish a generalized and systematic theory, although it is struggling to research and develop low-fouling films.

  The present inventors have conducted intensive research on the relationship between components in natural water and fouling of hollow fiber membranes. Among various natural water components, high molecular weight humic substances are the main components of membrane fouling. Non-Patent Document 1 clarifies that it is one of the causative substances. The humic substance is “a general term for macromolecular organic acids that are chemically and biologically synthesized from the degradation products of plant remnants and microorganisms and decomposed by microorganisms,” according to Non-Patent Document 2. The molecular size of humic substances can be roughly classified into three types: high molecular humic substances, medium molecular humic substances, and low molecular humic substances. It is also stated that the molecular diameter of the polymer humic substance according to the Stokes-Einstein equation is approximately larger than 0.1 μm and smaller than 0.45 μm. That is, it is extremely effective to reduce fouling of the hollow fiber membrane in membrane filtration using natural water as treated water, making it difficult to attach a high molecular humic substance having a molecular diameter of 0.1 to 0.45 μm to the membrane. It can be said that it is an effective means.

  Also disclosed is an invention that focuses on reducing membrane fouling in a polyvinylidene fluoride resin separation membrane. For example, Patent Document 1 discloses a technique for improving stain resistance by a hollow fiber membrane in which the inner and outer surfaces of a hollow fiber membrane made of polyvinylidene fluoride resin and the surface of the micropores are coated with an ethylene-vinyl alcohol copolymer. . In Patent Document 1, it is stated that the smaller the average pore size, the greater the water permeability resistance, which is not preferable because the water permeability performance of the membrane is lowered, and it is not preferable because the performance to prevent turbid substances and the like is reduced when it is larger, The relationship between the hole diameter and fouling is not discussed at all. In addition, the average pore diameter of the hollow fiber membrane that is experimentally clarified in Patent Document 1 is 0.19 μm even at the minimum, and the performance of the hollow fiber membrane having an average pore diameter smaller than that is experimentally clarified. The current situation is that no detailed examination has been made.

  Further, it is qualitatively said that the stain resistance is improved if the membrane is made hydrophilic, and various hydrophilization means are disclosed. For example, Patent Document 2 discloses a method for modifying a membrane surface by hydrophilization treatment of an aqueous polyvinylidene fluoride resin with an alkaline solution, but there is no discussion about the soiling property of the membrane, and particularly with respect to the pore size. The relationship is not described at all.

  In Patent Document 3, a hollow fiber membrane made of polyvinylidene fluoride having a predetermined average pore diameter in the surface layer is disclosed, but the average pore diameter of the hollow fiber membrane experimentally clarified in Patent Document 3 is the smallest. However, the performance of the hollow fiber membrane having an average pore diameter of less than 0.26 μm has not been experimentally clarified, and no detailed study has been made. Further, membrane fouling and average pore diameter The relationship with is not mentioned at all.

  Patent Document 4 discloses a hollow fiber membrane made of polyvinylidene fluoride having a dense structure layer having a maximum pore size of 0.037 μm or more and 0.040 μm or less. However, it is a hollow for medical use intended to surely remove viruses. Since it is a thread membrane and the surface of the water to be treated is a coarse structure layer having a pore diameter of 0.29 μm or more and 0.44 μm or less, it is considered that the polymer humic substance is blocked and fouling is accelerated.

As described above, hollow fiber membranes made of polyvinylidene fluoride resin having an average pore diameter of 0.1 μm or less on the surface of the water to be treated aimed at reducing fouling due to polymer humic substances that are the main causative substances of the membrane have been known so far. There wasn't.
Understanding the fouling substance of hollow fiber UF membrane in water purification treatment, Journal of Water Supply Association, Vol.71, No.5, pp.2-13 (2002). Mechanism of membrane fouling in river water UF membrane filtration, Journal of Water Supply Association, Vol.69, No.2, pp.12-23 (2000). Japanese Patent Application Laid-Open No. 2002-233739 JP 58-93734 A Japanese Patent No. 2899903 International Publication WO03 / 026679 Pamphlet

  The subject of the present invention is a hollow fiber membrane made of polyvinylidene fluoride resin for the purpose of filtration of natural water. Is to obtain a long-life hollow fiber membrane.

As a result of intensive studies to solve the above problems, the inventors have invented a hollow fiber membrane made of a high-performance polyvinylidene fluoride resin in which the decrease in water permeability due to contamination of natural water components is small. That is, the present invention
(1) In the hollow fiber membrane made of polyvinylidene fluoride resin, the average pore diameter of the pores on the treated water side surface of the hollow fiber membrane is 0.01 μm or more and 0.1 μm or less, and on the treated water side surface The pore density is 1 × 10 ¹³ / m ² or more and 1 × 10 ¹⁵ / m ² or less, and the porosity on the surface to be treated is 3% or more and 30% or less. Hollow fiber membrane.

(2) Pure water permeation performance at 100 kPa and 25 ° C. is 0.4 m ³ / m ² · hr or more and 8 m ³ / m ² · hr or less, breaking strength is 3 N / piece or more and 30 kg / piece or less, and elongation at break The hollow fiber membrane having a degree of 30% to 300%.

  (3) The said hollow fiber membrane whose to-be-processed water side surface is an outer surface of a hollow fiber membrane.

  (4) A hollow fiber membrane module using a hollow fiber membrane.

  (5) A membrane filtration device comprising the hollow fiber membrane module.

(6) A water treatment method for obtaining permeate from treated water using the membrane filtration device.
It consists of

  A hollow fiber membrane made of a high-performance polyvinylidene fluoride resin that has excellent chemical durability and physical strength, has little deterioration in water permeability due to contamination of natural water components, and has high stain resistance. In addition, it is possible to provide a membrane filtration device that can reduce the water production cost.

The hollow fiber membrane of the present invention has an average pore diameter of 0.01 μm or more and 0.1 μm or less, preferably 0.01 μm or more and 0.08 μm, more preferably 0.02 μm or more and 0.05 μm or less on the surface to be treated. And the pore density on the surface to be treated is 1 × 10 ¹³ / m ² or more and 1 × 10 ¹⁵ / m ² or less, preferably 2 × 10 ¹³ / m ² or more and 8 × 10 ¹⁴ / M ² or less, more preferably 3 × 10 ¹³ pieces / m ² or more and 6 × 10 ¹⁴ pieces / m ² or less, and the porosity on the treated water side surface is 3% or more and 30% or less, preferably The pores are 4% or more and 28% or less, more preferably 5% or more and 25% or less. An example of the hollow fiber membrane of the present invention having such pores is shown in FIG. FIG. 1 is a surface photograph of a hollow fiber membrane taken with a scanning electron microscope. Black portions 1 are pores present on the surface to be treated of the hollow fiber membrane of the present invention.

  The treated water defined in the present invention is raw water supplied to the membrane in hollow fiber membrane filtration.For example, in the water purification treatment field, natural water such as river water, lake water, and groundwater is mainly treated water. Become. The treated water may be sewage secondary treated water or seawater.

  In the case of the outer surface, the surface defined by the present invention is the surface when the surface of the hollow fiber membrane is viewed from directly above, and in the case of the surface of the inner surface, the hollow fiber membrane is cut and exposed. The surface when the inside of the hollow fiber membrane is viewed from directly above, the same structure may not be continuous in the thickness direction of the hollow fiber membrane, and the average pores on the treated water side surface The pore diameter, the pore density, and the aperture ratio may be measured with respect to pores (for example, a portion 1 in FIG. 1) grasped by observing the surface with a scanning electron microscope or the like. The surface to be treated as defined in the present invention is a surface where membrane supply water such as natural water first comes into contact with the hollow fiber membrane. Generally, as described above, a hollow fiber membrane has two surfaces, an outer surface and an inner surface. In the present invention, either surface can be configured as a surface to be treated, but generally has a large amount of turbidity such as natural water. In the case of filtering the contained water, the so-called external pressure type, in which raw water is passed from the outer surface side to the inner surface side of the hollow fiber membrane, easily discharges the dirt peeled off by physical cleaning to the outside of the membrane module system. Therefore, the surface to be treated is more preferably the outer surface of the hollow fiber membrane.

  The average pore diameter of the pores on the surface to be treated as defined in the present invention means that the surface of the water to be treated is photographed at a magnification of 10,000 times or more using a scanning electron microscope, 10 or more, preferably 20 It is the value obtained by measuring the diameter of the above-mentioned arbitrary pores and calculating the number average. When the pores on the surface to be processed are not circular, a method of obtaining a circle (equivalent circle) having an area equal to the area of the pores with an image processing apparatus or the like and setting the equivalent circle diameter as the diameter of the pores It can ask for.

  The present inventors have prepared three types of Millipore Durapore membrane filters, VVLP (hydrophilic polyvinylidene fluoride resin flat membrane, pore size 0.1 μm), GVWP (hydrophilic polyvinylidene fluoride resin flat membrane, pore size 0.22 μm). ), VVHP (hydrophobic polyvinylidene fluoride resin flat membrane, pore size 0.1 μm), the relationship between the soiling property of the membrane and the hydrophilic / hydrophobic properties and the average pore size, the lake water at a constant pressure with a head difference of 1.8 m The experiment was conducted by filtering. The permeated water from the flat membrane cell using the above three kinds of flat membranes was received in a beaker without performing physical cleaning, and the filtration resistance transition was calculated from the change in the amount of permeated water over time, and the filtration resistance in the order of VVLP <VVHP <GVWP. Clarified that the rise is large. That is, it has been found that a phenomenon different from the conventional knowledge that a hydrophilic membrane has a mean pore diameter of 0.22 μm and is more easily soiled than a hydrophobic membrane having an average pore diameter of 0.1 μm occurs. That is, as described above, the present inventors believe that the molecular diameter of the polymer humic substance is 0.1 μm to 0.45 μm, and that the polymer humic substance is a main cause of membrane fouling. Therefore, even if the membrane is hydrophilic, if the average pore size of the pores on the membrane surface is the same as that of the polymer humic substance, the polymer humic substance is completely clogged by the pores on the membrane surface and fouling occurs. It was clarified that the fouling hardly occurs by making the film having a surface having an average pore diameter of 0.1 μm or less smaller than that of the polymer humic substance. However, in order to maintain practical water permeability, the average pore diameter needs to be 0.01 μm or more.

  As described above, the fouling resistance is improved by reducing the average pore diameter of the pores on the surface of the water to be treated, and it has been found that the decrease in water permeability performance due to the continued filtration of the water to be treated can be reduced. On the other hand, simply reducing the average pore size decreases the water permeation performance of pure water, increases the filtration differential pressure, and cannot perform stable operation for a long time when natural water is filtered.

Therefore, as a result of investigations such as improvement of the water permeability of the entire membrane, the present inventors have found that the number of pores, that is, the pore density on the surface to be treated is 1 × 10 ¹³ / m ² or more, and It has been found that continuous stable filtration of natural water cannot be performed unless the open area ratio on the treated water side surface is 3% or more. That is, by opening a large number of small pores, it is possible to reduce the proportion of pores that are temporarily clogged by depositing contaminants such as polymer humic substances on the film surface, and every 30 to 2 hours. It has been clarified that the filtration resistance between physical washings does not increase rapidly and that stable filtration operation can be continued. The film forming technique for increasing the pore density and the open area ratio is as described later. However, if the pore density on the surface of the water to be treated and the open area ratio are too high, there is a high possibility of hindering the maintenance of the physical strength and physical durability of the membrane surface. The density of pores on the surface is required to be 1 × 10 ¹⁵ holes / m ² or less and the porosity is 30% or less.

Here, the pore density on the treated water side surface defined in the present invention is a photograph using a scanning electron microscope or the like at a magnification at which pores can be clearly confirmed on the treated water side surface of the hollow fiber membrane. The number of pores in the photograph is counted and converted to the number of pores per 1 m ² is defined as the pore density. It is preferable to count and average over a wide area such as a plurality of areas. For example, in the examples of the present invention, the number of pores per 3 μm square of a scanning electron micrograph was counted and calculated. It is also possible to analyze a photograph with an image processing apparatus. Further, the open area ratio on the surface to be treated defined in the present invention is obtained by calculation from the following formula from the average pore diameter and the pore density.

Opening ratio (%) = π × (average pore diameter / 2) ² × (pore density) × 100.

  The polyvinylidene fluoride resin in the present invention is a resin containing a vinylidene fluoride homopolymer and / or a vinylidene fluoride copolymer. A plurality of types of vinylidene fluoride copolymers may be contained. The vinylidene fluoride copolymer is a polymer having a vinylidene fluoride residue structure, and is typically a copolymer of a vinylidene fluoride monomer and other fluorine-based monomers. Examples of the copolymer include a copolymer of vinylidene fluoride and at least one selected from vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluoroethylene chloride. As long as the effects of the present invention are not impaired, a monomer such as ethylene other than the fluorine-based monomer may be copolymerized. In addition, for the purpose of enhancing the stain resistance, it is selected from cellulose ester, fatty acid vinyl ester, vinyl pyrrolidone, ethylene oxide, acrylonitrile, vinyl alcohol, etc. within a range that does not impair the chemical durability and physical strength of the polyvinylidene fluoride resin. It may contain a hydrophilic polymer having at least one kind. The weight average molecular weight of the polyvinylidene fluoride resin may be appropriately selected depending on the required strength and water permeability of the hollow fiber membrane, but is preferably in the range of 100,000 to 800,000. In consideration of processability to a hollow fiber membrane, the range of 200,000 to 600,000 is more preferable, and the range of 250,000 to 500,000 is more preferable.

The hollow fiber membrane of the present invention preferably has a pure water permeation performance at 100 kPa and 25 ° C. in the range of 0.4 m ³ / m ² · hr to 8 m ³ / m ² · hr, more preferably 0.6 m. ³ / m ² · hr or more and 6 m ³ / m ² · hr or less, more preferably 0.8 m ³ / m ² · hr or more and 5 m ³ / m ² · hr or less, and the breaking strength is 3 N / piece or more and 30 N / N or less, more preferably 4 N / line or more and 25 N / line or less, more preferably 5 N / line or more and 20 N / line or less, and the elongation at break is 30% or more and 300% or less. It is preferable to be in the range, preferably 40% to 250%, more preferably 50% to 200%. By being in this range, it is possible to obtain a hollow fiber membrane that exhibits sufficient water permeability under normal use conditions and that does not break. In addition, the pure water permeation performance of the hollow fiber membrane is a small module (about 20 cm in length, about 1 to 10 hollow fiber membranes) using reverse osmosis membrane treated water as a driving force with a water level difference of 1.5 m at 25 ° C. The value obtained by measuring the amount of permeated water for a certain time is calculated by converting per 100 kPa. The water permeation performance may be obtained by converting a value obtained by pressurizing to a constant pressure with a pump or the like per 100 kPa. The water temperature may also be measured at a temperature other than 25 ° C. and converted to a value at 25 ° C. from the viscosity of the evaluation liquid. The tensile strength at break is determined by measuring a load of a full scale of 2000 g or 5000 g at a test length of 50 mm 10 times at a crosshead speed of 50 mm / min and averaging the results.

  The above-described hollow fiber membrane is accommodated in a casing having a raw solution inlet and a permeate outlet and is used as a hollow fiber membrane module. Bundle multiple hollow fiber membranes into a cylindrical container and fix both ends or one end with polyurethane or epoxy resin to collect permeate, or fix both ends of the hollow fiber membrane in a flat plate shape for permeation Allow water to be recovered. In addition, it is preferable that the hollow fiber membrane module of this invention arrange | positions the to-be-processed water side surface of the said hollow fiber membrane on the to-be-processed water side, and is comprised. That is, when obtaining the permeated water from the inside of the hollow fiber membrane by bringing the water to be treated into contact with the outer surface of the hollow fiber membrane, the outer surface is a hollow fiber membrane module disposed as the surface to be treated, When permeate water is obtained from the outside of the hollow fiber membrane by bringing the water to be treated into contact with the inner surface of the hollow fiber membrane, the hollow fiber membrane module is preferably disposed with the inner surface as the surface to be treated.

  Further, the hollow fiber membrane module described above is used as a membrane filtration device for producing permeated water by providing a pressurizing means at least on the stock solution side (treated water side) or a suction means on the permeate side (permeate water side). It is done. A pump may be used as the pressurizing means, or a pressure due to a water level difference may be used. Moreover, what is necessary is just to utilize a pump and a siphon as a suction means. By supplying water to be treated such as surface water, lake water, and groundwater to such a membrane filtration device, it is possible to reduce the cost of producing permeated water.

  The hollow fiber membrane of the present invention is carried out, for example, by the production method described below.

  A polyvinylidene fluoride resin solution is dissolved at a relatively high temperature in a poor solvent or a good solvent of the polyvinylidene fluoride resin at a relatively high concentration of about 20 wt% to 60 wt% or less. The polyvinylidene fluoride resin solution is prepared and phase-separated by cooling and solidifying to form a hollow fiber membrane having a spherical structure. If the concentration of the polyvinylidene fluoride resin is increased, a hollow fiber membrane having high strength and elongation can be obtained. However, if the concentration is too high, the porosity of the hollow fiber membrane is reduced and the permeation performance is lowered. Further, if the viscosity of the polyvinylidene fluoride resin solution is not within an appropriate range, it is difficult to handle and it becomes impossible to form a film.

  Here, the poor solvent means that the polyvinylidene fluoride resin cannot be dissolved by 5% by weight or more at a low temperature of 60 ° C. or less, but it is 60 ° C. or more and the melting point of the polyvinylidene fluoride resin or less (for example, the polymer is It is a solvent that can be dissolved in an amount of 5% by weight or more in a high temperature region of about 178 ° C. when it is composed of vinylidene fluoride homopolymer alone. A solvent capable of dissolving 5% by weight or more of a polyvinylidene fluoride resin in a low temperature region of 60 ° C. or lower with respect to a poor solvent is a good solvent, up to the melting point of the polyvinylidene fluoride resin or the boiling point of the solvent. A solvent that does not dissolve or swell the resin is defined as a non-solvent. In the case of a polyvinylidene fluoride resin hollow fiber membrane, as a poor solvent, cyclohexanone, isophorone, γ-butyrolactone, methyl isoamyl ketone, dimethyl phthalate, propylene glycol methyl ether, propylene carbonate, diacetone alcohol, glycerol triacetate, etc. Long alkyl ketones, esters, glycol esters, organic carbonates and the like, and mixed solvents thereof. Even a mixed solvent of a non-solvent and a poor solvent is defined as a poor solvent if it satisfies the definition of the poor solvent. Examples of good solvents include N-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethylacetoxide, dimethylformamide, methyl ethyl ketone, acetone, tetrahydrofuran, tetramethylurea, trimethyl phosphate, and other lower alkyl ketones, esters, amides, and the like. The mixed solvent is mentioned. In cooling and solidifying the polyvinylidene fluoride resin solution, a method of discharging the polyvinylidene fluoride resin solution from the outer tube of the double tube die into the cooling bath is preferable. At this time, the cooling liquid used in the cooling bath is preferably solidified using a liquid containing a poor solvent or a good solvent having a temperature of 5 to 50 ° C. and a concentration of 60 to 100% by weight. The cooling liquid may contain a non-solvent in addition to the poor solvent and the good solvent. However, when a liquid containing a non-solvent as a main component is used as the cooling liquid, the phase caused by non-solvent infiltration rather than phase separation by cooling solidification is used. Separation is prioritized, making it difficult to obtain a spherical structure. Moreover, it is set as a hollow fiber membrane by solidifying in a cooling bath, discharging a hollow part formation fluid from the pipe | tube inside a double tube | pipe type nozzle | cap | die. At this time, a gas or a liquid can be normally used as the hollow portion forming fluid, but in the present invention, a liquid containing a poor solvent or a good solvent having a concentration similar to that of the cooling liquid of 60 wt% to 100 wt%. Can be preferably used. Examples of the non-solvent include water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, and pentane. Aliphatic hydrocarbons such as diol, hexanediol, low molecular weight polyethylene glycol, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromatic polyhydric alcohols, chlorinated hydrocarbons, or other chlorinated organic liquids and mixtures thereof A solvent is mentioned.

  Next, a three-dimensional network structure obtained by a non-solvent induced phase separation method is laminated on the polyvinylidene fluoride resin hollow fiber membrane having a spherical structure obtained as described above. In other words, a polyvinylidene fluoride resin hollow fiber membrane having a spherical structure is coated on a polyvinylidene fluoride resin solution that causes non-solvent-induced phase separation, and then immersed in a coagulation bath to form a layer having a three-dimensional network structure. It is a method of laminating. Here, in the polyvinylidene fluoride resin solution for forming a three-dimensional network structure, as shown below, the average pore diameter on the treated water side is reduced and the pore density and the porosity are increased. It is a feature of the present invention that a surfactant is added as a pore opening agent.

  That is, the above-mentioned polyvinylidene fluoride resin solution for forming the three-dimensional network structure is composed mainly of a polyvinylidene fluoride resin and a good solvent, and there are an average pore diameter, pore density, In order to change the porosity, another component called a pore opening agent is added. Although it is known in the prior art to add components other than the polymer and the good solvent to the polymer solution in the non-solvent induced phase separation method, the feature of the present invention is that at least one surfactant is added to the pore opening agent. Is to add. Since the surfactant has a hydrophilic part and a hydrophobic part, it is dispersed microscopically in the polyvinylidene fluoride resin and solvent and other components, and after coagulation and washing, it elutes with the solvent from the membrane to form fine pores. Many are formed. When the addition amount of the surfactant is increased, the stability of the polyvinylidene fluoride resin solution is gradually impaired, and the polyvinylidene fluoride resin solution is gelled (the polyvinylidene fluoride resin is phase-separated / insolubilized). As long as the stability of the polyvinylidene fluoride resin solution is not impaired, it is preferable to add an excess of the surfactant exceeding the critical micelle concentration because the effect of reducing the surface tension of the surfactant is maximized. By adding this excessive surfactant, the micelle becomes a starting point for microphase separation for forming a fine structure, and a small porous structure is formed. Further, for the purpose of controlling the phase separation / solidification of the entire three-dimensional stitch structure, it is also preferable to add an organic compound and / or an inorganic compound and / or the aforementioned non-solvent in addition to the surfactant. As the organic compound, those that are soluble in both the solvent used for the polyvinylidene fluoride resin solution and the non-solvent that causes non-solvent-induced phase separation are preferably used. Examples thereof include water-soluble polymers such as polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol, polyethylene imine, polyacrylic acid, and dextran, glycerin, and saccharides. As the inorganic compound, those that are soluble in both the solvent used for the fluororesin polymer solution and the non-solvent that causes non-solvent-induced phase separation are preferable. Examples include calcium chloride, magnesium chloride, lithium chloride, and barium sulfate. Can do. For the purpose of further improving the stain resistance, it is also possible to preferably employ a hydrophilic polymer having at least one selected from cellulose ester, fatty acid vinyl ester, vinyl pyrrolidone, ethylene oxide, acrylonitrile, vinyl alcohol and the like.

  The concentration of the polyvinylidene fluoride resin in the polyvinylidene fluoride resin solution is usually preferably 5 to 30% by weight, more preferably 10 to 20% by weight. If it is less than 5% by weight, the physical strength of the three-dimensional network structure layer is lowered, and if it exceeds 30% by weight, the transmission performance is lowered. The dissolution temperature of the polyvinylidene fluoride resin solution varies depending on the concentration of the polyvinylidene fluoride resin, the type of solvent, and the type and concentration of additives such as surfactants, but the dissolution temperature is approximately 40 ° C to 120 ° C. It is. It is preferable to heat for several hours while stirring at a temperature below the boiling point of the solvent to make a transparent solution.

  After applying the polyvinylidene fluoride resin solution prepared as described above onto the polyvinylidene fluoride resin hollow fiber membrane having the spherical structure, it is immersed in a coagulation bath containing a non-solvent as a main component to form a three-dimensional network. A layer having a structure is stacked. The hollow fiber membrane of the present invention can be obtained by the film forming method as described above.

  The present invention will be described below with reference to specific examples, but the present invention is not limited to these examples.

The average pore diameter of the hollow fiber membranes in the examples was obtained by photographing the outer surface of the hollow fiber, which is the surface to be treated, at a magnification of 30,000 using a scanning electron microscope (S-800) (manufactured by Hitachi, Ltd.). The pore diameters of the arbitrary pores were measured and obtained by number averaging. In addition, the pore density was calculated by counting the number of pores per 3 μm square in the photograph for which the average pore diameter was obtained, and converting it to the number of pores per 1 m ² . The open area ratio was calculated from the calculated average pore diameter and pore density by the following formula.

Opening ratio (%) = π × (average pore diameter / 2) ² × (pore density) × 100.

The pure water permeation performance was determined as follows. First, a miniature module having a length of 200 mm consisting of four hollow fiber membranes was prepared, and the external pressure total filtration of reverse osmosis membrane filtered water was performed for 5 minutes under the conditions of a temperature of 25 ° C. and a filtration differential pressure of 16 kPa. (M ³ ) was determined. Next, the permeation amount (m ³ ) is converted into a value per unit time (h) and effective membrane area (m ² ), and further converted to a value at a pressure of 100 kPa by multiplying by (100/16). Pure water permeation performance was determined.

  The breaking strength and breaking elongation were measured using a tensile testing machine (TENSILON / RTM-100) (manufactured by Toyo Baldwin) at a tensile speed of 50 mm / min. Ten measurements were performed on one type of hollow fiber membrane, and the number average and the breaking strength and breaking elongation were determined.

The stain resistance was determined by the following drivability evaluation. Using a hollow fiber membrane module prepared to have a diameter of 3 cm, a length of 50 cm, and an effective membrane area of 0.3 m ² , a constant flow rate external pressure total filtration operation was performed on Lake Biwa water at a membrane filtration flow rate of 3 m / d. Every 60 minutes, backwashing with a 5 ppm sodium hypochlorite aqueous solution was performed for 30 seconds, and air scrubbing with air was performed for 1 minute. The change in filtration pressure difference during one month was measured, and the daily average filtration pressure increase rate (kPa / day) was calculated and compared using the following formula.

Filtration differential pressure increase rate (kPa / day) = (P ₁ −P ₀ ) / T.

Here, the filtration differential pressure immediately after the physical cleaning at the start of the operation evaluation is P ₀ (kPa), the filtration differential pressure at the end of the operation evaluation is P ₁ (kPa), and the operation period is T (day). If the filtration differential pressure increase rate is about 0.35 kPa / day or less, the filtration differential pressure increase is about 60 kPa after 6 months of operation. Considering that the initial filtration differential pressure is about 20 to 30 kPa and the standard for chemical cleaning is about 100 to 150 kPa, the filtration differential pressure increase rate is 0.35 kPa / day. Thus, it can be said that the hollow fiber membrane has high stain resistance and can be stably operated.

<Example 1>
A vinylidene fluoride homopolymer having a weight average molecular weight of 41,000 and γ-butyrolactone were dissolved at a temperature of 170 ° C. at a ratio of 36% by weight and 64% by weight, respectively. The polymer solution is discharged from the die with accompanying γ-butyrolactone as a hollow portion forming liquid, and cooled, phase separated, and solidified / solidified in a cooling bath made of an aqueous γ-butyrolactone to produce a hollow fiber membrane having a spherical structure. did.

  A vinylidene fluoride homopolymer having a weight average molecular weight of 284,000, polyoxyethylene coconut oil sorbitan and dimethyl sulfoxide were mixed at a ratio of 13% by weight, 5% by weight and 82% by weight, respectively, and dissolved at a temperature of 100 ° C. . This polymer solution was applied to the outer surface of the hollow fiber membrane, solidified with water at 30 ° C., and then desolvated by washing with water to obtain the hollow fiber membrane of the present invention.

The average pore diameter of the pores on the outer surface of the obtained hollow fiber membrane, that is, the surface to be treated is 0.03 μm, the pore density is 8.0 × 10 ¹³ pores / m ² , the porosity is 5.7%, The water permeation performance was 0.8 m ³ / m ² · hr, the breaking strength was 10.8 N / piece, and the breaking elongation was 115%. The rate of increase in filtration resistance of Lake Biwa water was 0.22 kPa / day, and the soil resistance was high, and stable filtration operation could be performed.

<Example 2>
A vinylidene fluoride homopolymer having a weight average molecular weight of 284,000, polyoxyethylene sorbitan monooleate, and dimethyl sulfoxide were mixed at a ratio of 13 wt%, 5 wt%, and 82 wt%, respectively, and dissolved at a temperature of 100 ° C. did. This polymer solution was applied to the outer surface of the same hollow fiber membrane as in Example 1, solidified with water at 35 ° C., and then desolvated by washing with water to obtain the hollow fiber membrane of the present invention.

The average pore diameter of the pores on the outer surface of the obtained hollow fiber membrane, that is, the surface to be treated is 0.04 μm, the pore density is 9.3 × 10 ¹³ pores / m ² , the open area ratio is 11.7%, The pure water permeation performance was 1.6 m ³ / m ² · hr, the breaking strength was 10.8 N / piece, and the breaking elongation was 113%. The rate of increase in filtration resistance of Lake Biwa water was 0.31 kPa / day, and the soil resistance was high, and stable filtration operation could be performed.

<Example 3>
A vinylidene fluoride homopolymer having a weight average molecular weight of 284,000, water, N-methyl-2-pyrrolidone, and polyoxyethylene coconut oil sorbitan in a proportion of 14% by weight, 1% by weight, 80% by weight and 5% by weight, respectively. And dissolved at a temperature of 90 ° C. This polymer solution was applied to the outer surface of the same hollow fiber membrane as in Example 1, solidified with water at 30 ° C., and then desolvated by washing with water to obtain the hollow fiber membrane of the present invention.

The average pore diameter of the pores on the outer surface of the obtained hollow fiber membrane, that is, the surface to be treated is 0.05 μm, the pore density is 1.1 × 10 ¹⁴ / m ² , the open area ratio is 21.6%, The pure water permeation performance was 1.6 m ³ / m ² · hr, the breaking strength was 9.8 N / piece, and the breaking elongation was 108%. The rate of increase in filtration resistance of Lake Biwa water was 0.20 kPa / day, and the soil resistance was high, and stable filtration operation could be performed.

<Example 4>
A vinylidene fluoride homopolymer having a weight average molecular weight of 284,000, polyoxyethylene sorbitan monooleate, water and dimethylformamide were mixed at a ratio of 11% by weight, 5% by weight, 2% by weight and 82% by weight, respectively. Dissolved at a temperature of 100 ° C. This polymer solution was applied to the outer surface of a hollow fiber membrane similar to that in Example 1, solidified with water at 40 ° C., and then desolvated by washing with water to obtain the hollow fiber membrane of the present invention.

The average pore diameter of the pores on the treated water side surface of the obtained hollow fiber membrane was 0.08 μm, the pore density was 2.1 × 10 ¹³ pores / m ² , the open area ratio was 10.6%, and pure water permeation The performance was 1.6 m ³ / m ² · hr, the breaking strength was 11.8 N / piece, and the breaking elongation was 105%. The rate of increase in filtration resistance of Lake Biwa water was 0.34 kPa / day, and the soil resistance was high, and a stable filtration operation could be performed.

<Example 5>
A vinylidene fluoride homopolymer having a weight average molecular weight of 284,000, cellulose acetate, N-methyl-2-pyrrolidone and polyoxyethylene sorbitan monooleate were 15% by weight, 1% by weight, 77% by weight and 7% by weight, respectively. And dissolved at a temperature of 110 ° C. This polymer solution was applied to the outer surface of a hollow fiber membrane similar to that in Example 1, solidified with water at 25 ° C., and then desolvated by washing with water to obtain the hollow fiber membrane of the present invention.

The average pore diameter of the pores on the treated water side surface of the obtained hollow fiber membrane was 0.01 μm, the pore density was 6.8 × 10 ¹⁴ pores / m ² , the open area ratio was 5.3%, and pure water permeation was achieved. The performance was 0.9 m ³ / m ² · hr, the breaking strength was 10.8 N / piece, and the breaking elongation was 112%. The rate of increase in filtration resistance of Lake Biwa water was 0.25 kPa / day, and the soil resistance was high, and stable filtration operation could be performed.

<Example 6>
A vinylidene fluoride homopolymer having a weight average molecular weight of 284,000, polyoxyethylene coconut oil sorbitan and dimethyl sulfoxide were mixed at a ratio of 13% by weight, 4% by weight and 83% by weight, respectively, and dissolved at a temperature of 100 ° C. . This polymer solution was applied to the outer surface of the same hollow fiber membrane as in Example 1, solidified with water at 35 ° C., and then desolvated by washing with water to obtain the hollow fiber membrane of the present invention.

The average pore diameter of the pores on the treated water side surface of the obtained hollow fiber membrane was 0.03 μm, the pore density was 5.2 × 10 ¹³ pores / m ² , the open area ratio was 3.7%, and pure water permeation was achieved. The performance was 0.8 m ³ / m ² · hr, the breaking strength was 11.8 N / piece, and the breaking elongation was 112%. The rate of increase in filtration resistance of Lake Biwa water was 0.30 kPa / day, and the soil resistance was high, and stable filtration operation could be performed.

<Comparative Example 1>
A vinylidene fluoride homopolymer having a weight average molecular weight of 284,000, a polyethylene glycol having a molecular weight of 20,000, water and dimethylformamide were mixed in a ratio of 13% by weight, 5% by weight, 3% by weight and 79% by weight, respectively. Dissolved at a temperature of ° C. This polymer solution was applied to the outer surface of a hollow fiber membrane similar to that in Example 1, solidified with water at 60 ° C., and then desolvated by washing with water to obtain the hollow fiber membrane of the present invention.

The average pore diameter of the pores on the treated water side surface of the obtained hollow fiber membrane is 0.4 μm, the pore density is 7.1 × 10 ¹¹ pores / m ² , the aperture ratio is 8.9%, and pure water permeation is performed. The performance was 1.8 m ³ / m ² · hr, the breaking strength was 9.8 N / piece, and the breaking elongation was 103%. The rate of increase in filtration resistance of Lake Biwa water was 350 kPa / day, and the stain resistance was remarkably poor, and the filtration operation could only be performed for several hours.

<Comparative example 2>
A vinylidene fluoride homopolymer having a weight average molecular weight of 284,000 and dimethyl sulfoxide were mixed at a ratio of 15 wt% and 85 wt%, respectively, and dissolved at a temperature of 100 ° C. This polymer solution was applied to the outer surface of a hollow fiber membrane similar to that in Example 1, solidified with water at 60 ° C., and then desolvated by washing with water to obtain the hollow fiber membrane of the present invention.

The average pore diameter of the pores on the treated water side surface of the obtained hollow fiber membrane is 0.03 μm, the number of pores is 3.7 × 10 ¹³ pores / m ² , the open area ratio is 2.6%, and the pure water permeation performance is 0.5 m ³ / m ² · hr, breaking strength was 11.8 N / piece, and breaking elongation was 122%. The filtration resistance increase rate of Lake Biwa water was 0.95 kPa / day, and the soil resistance was poor.

<Comparative Example 3>
A vinylidene fluoride homopolymer having a weight average molecular weight of 284,000, water and dimethylformamide were mixed at a ratio of 12% by weight, 3% by weight and 85% by weight, respectively, and dissolved at a temperature of 100 ° C. This polymer solution was applied to the outer surface of a hollow fiber membrane similar to that in Example 1, solidified with water at 40 ° C., and then desolvated by washing with water to obtain the hollow fiber membrane of the present invention.

The average pore diameter of the pores on the treated water side surface of the obtained hollow fiber membrane was 0.09 μm, the number of pores was 6.7 × 10 ¹² / m ² , the open area ratio was 4.3%, and the water permeability was 1 .6m ³ / ^m 2 · hr, breaking strength is 9.8 N / present, elongation at break was 103%. The rate of increase in filtration resistance of Lake Biwa water was 4.5 kPa / day, indicating poor soil resistance, and the chemical cleaning period was reached after 3 weeks of filtration operation.

  The present invention provides a high-performance hollow fiber membrane that uses a polyvinylidene fluoride resin excellent in chemical durability and physical strength, is resistant to contamination and maintains high water permeability for a long time. The hollow fiber membrane of the present invention can be used for a membrane filtration device that exhibits low water production costs in water treatment fields such as drinking water production, water purification treatment, sewage, wastewater treatment, and seawater desalination pretreatment.

It is an example of the to-be-processed water side surface photograph of the hollow fiber membrane of this invention.

Explanation of symbols

  1 pore

Claims (6)

In the hollow fiber membrane made of polyvinylidene fluoride resin, the average pore diameter of the hollow fiber membrane on the treated water side surface is 0.01 μm or more and 0.1 μm or less, and the pore density on the treated water side surface 1 × 10 ¹³ pieces / m ² or more and 1 × 10 ¹⁵ pieces / m ² or less, and the porosity of the treated water side surface is 3% or more and 30% or less. film. Pure water permeation performance at 100 kPa and 25 ° C. is 0.4 m ³ / m ² · hr or more and 8 m ³ / m ² · hr or less, breaking strength is 3 N / piece or more and 30 N / piece or less, and elongation at break is 30 The hollow fiber membrane according to claim 1, wherein the hollow fiber membrane is not less than 300% and not more than 300%. The hollow fiber membrane according to claim 1 or 2, wherein the treated water side surface is an outer surface of the hollow fiber membrane. The hollow fiber membrane module which uses the hollow fiber membrane in any one of Claims 1-3. A membrane filtration apparatus comprising the hollow fiber membrane module according to claim 4. A water treatment method for obtaining permeate from treated water using the membrane filtration device according to claim 5.