JP2011012242A - Manufacturing method of drawn resin porous membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resin porous membrane useful as a separation membrane such as a liquid filtrating membrane and a battery separator while maintaining a high liquid permeability by preventing decline of porosity in a dense layer in the vicinity of the surface layer controlling the separation performance at the time of a drawing operation for reinforcement of the mechanical strength, increase of the porosity, and the like.SOLUTION: The manufacturing method of a drawn resin porous membrane comprises a step of drawing a resin porous film in a state selectively wetted with a wetting liquid by 5 μm or more from its outer surface and 1/2 of the membrane thickness or less.

Description

  The present invention relates to a porous resin membrane used for various applications including a microfiltration membrane or a separator porous membrane such as a separator for an electrochemical element such as a battery or an electric double layer capacitor. The present invention relates to improvement of a method for producing a stretched resin porous membrane having improved rate and the like.

  Conventionally, various resin porous membranes, especially synthetic resin-based porous membranes, are widely used as separation membranes for gas diaphragm separation, gas-liquid separation, solid-liquid separation, etc., or as insulating materials, heat insulating materials, sound insulation materials, heat insulating materials, etc. Has been used. Among these, particularly when used as a separation membrane, the following characteristics that affect the separation function are required. First, it must have an appropriate porosity for separation efficiency of the porous membrane, have a uniform pore size distribution for the purpose of improving separation accuracy, and have an optimum pore size for the separation object Desired. Further, as the properties of the membrane constituent material, chemical resistance, weather resistance, heat resistance, strength and the like with respect to the characteristics of the separation object are required. Furthermore, sufficient elongation at break and stress at break are required as mechanical strength when using the porous membrane.

  As the porous membrane material resin, a polyolefin resin is often used (for example, Patent Documents 1 and 2), and in recent years, a vinylidene fluoride resin that is more excellent in chemical resistance, weather resistance, and heat resistance is also widely used. (Patent Documents 3 to 10). These methods for producing a porous resin membrane often include a stretching step for the purpose of increasing mechanical strength or porosity in the latter half (for example, Patent Documents 1, 2, and 7 to 10).

  There are various aspects of the method for producing a porous resin membrane including a stretching step. For example, after molding a mixture of a resin and a pore-forming agent into a membrane, this membrane-like molded product is obtained by (a) pore-forming agent. Stretching in a state of containing (Patent Document 9), (b) Substituting the pore-forming agent with an extraction solvent or a coagulation solvent, and then stretching in a state of containing the solvent (Patent Document 10), (c) There have been proposed methods such as substitution with an extraction solvent or a coagulation solvent, then removing the solvent, and then stretching in a dry state (Patent Documents 7, 8, and 9). In the case of the above (a) and (b), during stretching, the film-shaped molded body undergoes a relatively large dimensional shrinkage in a direction other than the stretching direction and exhibits a stretching behavior that is almost similar to an equal volume deformation. While the effect of increasing the rate is small, in the case of the above (c), the stretching behavior of the film-shaped molded body shows a relatively small dimensional shrinkage in the direction other than the stretching direction and is almost close to expansion deformation. Therefore, the effect of increasing the porosity is large. In the case of (b), there is also a problem that the heat input of the relaxation process usually performed for subsequent dimensional stabilization becomes difficult due to the heat of vaporization of the solvent. In general, the dry stretching (c) is advantageous in order to increase the porosity. However, according to the study by the present inventors, in this dry stretching, although the average porosity of the entire porous membrane increases, there is a disadvantage that the porosity in the surface layer of the porous membrane does not increase to a desired level. It was issued.

Japanese Patent Publication No.46-40119 Japanese Patent Publication No. 50-2176 JP-A 63-296940 JP-A-3-215535 WO99 / 47593A WO03 / 031038A WO2004 / 081109A WO2005 / 099879A JP 2001-179062 A Japanese Patent Laid-Open No. 2003-210954

  An object of this invention is to give the improvement of the manufacturing method of the resin porous membrane mentioned above, especially the extending process.

  According to the study by the present inventors, particularly in dry stretching, the stretching stress acting on the resin porous film having pores already formed does not lead to the expansion of the pores in the surface layer, but through the thickness reduction, etc. It was found that it acts as a stress in the direction of shrinking the vacancies. Such a decrease in the surface layer porosity causes clogging of the surface layer when the resin porous membrane is used as a separation membrane or a decrease in liquid permeability of the entire membrane, and should be suppressed as much as possible. According to further studies by the present inventors, it belongs to the category of dry stretching in general, but by stretching the surface layer portion of the resin porous membrane that has already been formed in a wet state, it is essential for the above problem. It has been found that significant improvements can be obtained. That is, in the method for producing a resin porous membrane of the present invention, the resin porous membrane is stretched while being selectively wetted with a wetting liquid from the outer surface to a depth of 5 μm or more and 1/2 or less of the film thickness. It is characterized by this.

The schematic explanatory drawing of the apparatus used in order to evaluate the water permeability of the hollow fiber porous membrane obtained by the Example and the comparative example.

(Resin porous membrane)
The method for producing a resin porous membrane according to the present invention is basically characterized by a stretching process applied to a resin porous membrane that has already been formed in a dry state. There are no restrictions. The porous membrane is applicable regardless of whether it is a flat membrane or a hollow fiber membrane. The resin forming the porous film can be either a hydrophilic resin or a hydrophobic resin, and either a natural resin or a synthetic resin can be used. However, considering durability such as when the liquid to be treated when used as a separation membrane is an aqueous liquid, it is preferable that the resin be insoluble in water. Typical examples of such water-insoluble resins include polyolefin resins (for example, Patent Documents 1 and 2), polyvinylidene fluoride resins (for example, Patent Documents 3 to 10), polytetrafluoroethylene resins, and polysulfone resins. , Polyethersulfone resin (WO02 / 058828A1), polyvinyl chloride resin, polyarylene sulfide resin, polyacrylonitrile resin, cellulose acetate resin (Japanese Patent Laid-Open No. 2003-31133), etc. Used as

  Among these, application to a porous film made of vinylidene fluoride resin having both chemical resistance, weather resistance, and heat resistance is most preferable. Generally, a vinylidene fluoride resin porous membrane is obtained by cooling a mixture of (A) a vinylidene fluoride resin and an organic liquid material that is compatible at least at an elevated temperature, and thereby fluorinating the organic liquid material. A method of obtaining a porous film by causing phase separation from vinylidene resin and removing the organic liquid from the film-like molded body of vinylidene fluoride resin containing the organic liquid separated by phase extraction (thermally induced phase separation method) Patent Documents 5 to 9), or (B) A film-like molded body of a mixture of the vinylidene fluoride resin and the organic liquid is brought into contact with a non-solvent of the vinylidene fluoride resin that is compatible with the organic liquid. A method of forming a film-like molded body of a vinylidene fluoride resin containing a non-solvent by causing phase separation between the organic liquid and the vinylidene fluoride resin while replacing the organic liquid with a non-solvent (non-solvent) Melting Induced phase separation method; Patent Documents 3 and 10), or (C) After molding a mechanically kneaded product of vinylidene fluoride resin, organic liquid material and inorganic fine powder which are incompatible with this into a film In many cases, the method is obtained by extracting and removing an organic liquid and inorganic fine powder from the film-like molded body to obtain a porous film (Patent Document 4). The method of the present invention can be obtained through any of the above methods. The present invention can also be applied to the prepared porous membrane.

  As described above, the method of the present invention can be formed on both flat membranes and hollow fiber membranes. In general, in filtered water treatment, it is easy to increase the membrane area per filtration device. It is preferably formed as a flat film shape for an electrochemical element separator including a battery. Hereinafter, embodiments in which a vinylidene fluoride resin porous membrane mainly having a hollow fiber form is formed by a thermally induced phase separation method and the stretching method of the present invention is applied thereto will be sequentially described. It can be easily understood that the present invention can be applied to various forms and materials of porous resin membranes including a flat membrane formed according to the conventional method by changing the conditions.

(Vinylidene fluoride resin)
The vinylidene fluoride resin that is the main film material is a vinylidene fluoride homopolymer, that is, a polyvinylidene fluoride, a copolymer with other monomers copolymerizable with vinylidene fluoride, or a mixture thereof, and a weight average Those having a molecular weight of 600,000 to 1,200,000, more preferably 650,000 to 1,000,000, particularly preferably 700,000 to 900,000 are preferably used. When the weight average molecular weight is less than 600,000, when the proportion of the organic liquid is increased in order to obtain a high porosity, it becomes difficult to form a film by decreasing the viscosity, and is over 1.2 million It takes a long time to uniformly mix the vinylidene fluoride resin and the organic liquid.

  As the monomer copolymerizable with vinylidene fluoride, one or more of ethylene tetrafluoride, hexafluoropropylene, ethylene trifluoride, ethylene trifluoride chloride, vinyl fluoride and the like can be used. The vinylidene fluoride resin preferably contains 70 mol% or more of vinylidene fluoride as a structural unit.

  When using a method in which a phase separation is caused by cooling a film-like molded body of a molten mixture of a vinylidene fluoride resin and an organic liquid in the process of producing a porous membrane (that is, a thermally induced phase separation method) Since phase separation occurs due to the crystallization of the vinylidene resin, a porous film having a high porosity tends to be obtained by using a highly crystalline vinylidene fluoride resin. For this reason, it is preferable to use a homopolymer composed of 100 mol% of vinylidene fluoride in the thermally induced phase separation method.

  For the purpose of suppressing the formation of spherulites in the thermally induced phase separation method, the weight average molecular weight (Mw) is 450,000 to 1,000,000, preferably 490,000 to 900,000, more preferably 600,000 to 800,000. The vinylidene fluoride resin for matrix (PVDF-I) is 25 to 98% by weight, preferably 50 to 95% by weight, more preferably 60 to 90% by weight, and the weight average molecular weight (Mw) is a medium high molecular weight fluoride. Ultrahigh molecular weight vinylidene fluoride resin (PVDF-II) 2 for crystal property modification, which is 1.4 times or more of vinylidene resin and 1.5 million or less, preferably 1.4 million or less, more preferably 1.3 million or less. It is also preferable to add ˜75 wt%, preferably 5 to 50 wt%, more preferably 10 to 40 wt%. When the Mw of the ultra-high molecular weight vinylidene fluoride resin (PVDF-II) is less than 1.4 times the Mw of the medium high molecular weight resin (PVDF-I), it is difficult to sufficiently suppress the formation of spherulites. If it exceeds 10,000, it is difficult to uniformly disperse in the matrix resin. In addition, if the amount of the ultrahigh molecular weight vinylidene fluoride resin is less than 2% by weight, the effect of suppressing spherulite is not sufficient. On the other hand, if it exceeds 75% by weight, stable film formation is difficult due to the occurrence of melt fracture during spinning. Tend to be.

  From the viewpoint of the performance of the obtained porous membrane, in many applications, it is preferable to use a homopolymer composed of 100 mol% of vinylidene fluoride because of its high chemical resistance and mechanical strength.

  On the other hand, when the performance of the obtained porous membrane is required to be flexible or stretchable, or in battery separator applications, the temperature at which pores are automatically closed due to softening of the membrane upon overheating, that is, the shutdown temperature is set. When it is desired to reduce the above, it is possible to adjust these characteristics by copolymerization, and it is preferable to use a copolymer as the vinylidene fluoride resin.

  The relatively high molecular weight vinylidene fluoride resin as described above can be obtained by emulsion polymerization or suspension polymerization, particularly preferably suspension polymerization.

  In order to produce a porous resin membrane by a thermally induced phase separation method, an organic liquid is added to the above-mentioned vinylidene fluoride resin to form a raw material composition for film formation.

(Organic liquid)
The vinylidene fluoride-based resin porous film is mainly formed of the above-mentioned vinylidene fluoride-based resin. For the production thereof, an organic liquid as a pore forming agent is used in addition to the vinylidene fluoride-based resin. The organic liquid is any organic liquid (room temperature) that is compatible with the vinylidene fluoride resin at least at an elevated temperature and causes phase separation from the vinylidene fluoride resin by cooling or contact with a non-solvent. And those that are liquefied only at an elevated temperature). For the production of the porous membrane by the thermally induced phase separation method, in addition to the above-mentioned vinylidene fluoride resin, an organic liquid composed of a plasticizer is preferably used as the pore forming agent. As such an organic liquid, a monomeric plasticizer and a polymeric plasticizer for a vinylidene fluoride resin are preferably used, and have compatibility with the vinylidene fluoride resin at a melt kneading temperature. Those having the characteristics of (iii) are particularly preferably used. As a result, it is possible to maintain the porosity (A1) high while reducing the thickness of the dense layer close to the cooling side surface with a high total layer porosity (A0).

(I) The melt-kneaded product with the vinylidene fluoride resin is 6 ° C. or more lower than the crystallization temperature Tc (° C.) of the vinylidene fluoride resin alone, preferably 9 ° C. or more, more preferably 12 ° C. or more lower. Giving a crystallization temperature Tc ′ (° C.),
(Ii) Crystal melting enthalpy ΔH ′ (J / g) on the basis of vinylidene fluoride resin mass as measured with a differential scanning calorimeter (DSC) on a film-like molded body obtained by cooling and solidifying the melt-kneaded product As 5 J / g or more, preferably 10 J / g or more, more preferably 25 J / g or more, most preferably 50 J / g or more, and (iii) JIS K7117-2 (cone-plate type rotational viscometer used) The viscosity measured at a temperature of 25 ° C. according to the standard is 200 mPa · s to 1000 Pa · s, more preferably 400 mPa · s to 100 Pa · s, still more preferably 500 mPa · s to 50 Pa · s.
The higher the viscosity of the organic liquid, the smaller the pore diameter in the formed porous film.

  As a preferable example of the organic liquid having the above-mentioned properties, a (poly) ester composed of an aliphatic dibasic acid and a glycol, that is, at least one of polyester or ester (mono- or diglycol ester of an aliphatic dibasic acid), preferably Is a polyester plasticizer having both ends sealed with a monovalent aromatic carboxylic acid.

  The aliphatic dibasic acid component constituting the (poly) ester at the center of the polyester plasticizer is preferably an aliphatic dibasic acid having 4 to 12 carbon atoms, particularly 6 to 10 carbon atoms. Examples of such an aliphatic dibasic acid component include succinic acid, maleic acid, fumaric acid, glutamic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid and the like, and industrial easy availability. To adipic acid is most preferred. These aliphatic dibasic acids may be used alone or in combination of two or more.

  The glycol component constituting the (poly) ester at the center of the polyester plasticizer is preferably a glycol having 2 to 18 carbon atoms, particularly 3 to 10 carbon atoms, such as aliphatic dihydric alcohol or polyalkylene glycol. Is used. These glycols may be used alone or in combination of two or more.

  The above-mentioned polyester plasticizer is preferably sealed at its molecular chain end with an aromatic monovalent carboxylic acid. Two or more aromatic monovalent carboxylic acids may be used in combination, but benzoic acid is particularly preferred from the viewpoint of industrial availability.

    In the present invention, a monomeric plasticizer or a water-insoluble solvent can be used in combination with the polyester plasticizer as long as the entire organic liquid satisfies the above characteristics.

  By selecting such a preferable organic liquid, a large amount of the organic liquid can be added to the vinylidene fluoride resin having the preferable molecular weight characteristics as described above, and a molded product solidified by cooling after melt extrusion can be obtained. After separation into a vinylidene fluoride resin phase and an organic liquid phase and removal of the organic liquid phase in a later extraction step, the porosity of both the total layer porosity and the dense layer porosity (the measurement method will be described later) is high. A membrane is obtained.

(Composition)
The raw material composition for forming a porous film by the heat-induced phase separation method has an organic liquid content of at least 200 parts by volume, more preferably 300 parts by volume, and still more preferably 100 parts by volume of vinylidene fluoride resin. It is preferable to form a mixture of 400 parts by volume or more and an upper limit of 1000 parts by volume or less, more preferably 700 parts by volume or less. When using the above-mentioned polyester plasticizer as the organic liquid, in addition to this, a monomeric ester plasticizer, water-insoluble Can be added.

  If the amount of the organic liquid is too small, it is difficult to obtain the object porosity of the present invention, and if it is too large, the melt viscosity is excessively decreased. There is a possibility that the mechanical strength of the porous film to be produced is lowered.

  The addition amount of the organic liquid is within the above range so that the Tc ′ of the melt-kneaded product with the vinylidene fluoride resin is 120 to 140 ° C., more preferably 125 to 139 ° C., and further preferably 130 to 138 ° C. It is preferable to adjust.

(Mixing / melt extrusion)
As an example, when forming a film-shaped molded article by a thermally induced phase separation method, the melt-extruded composition melt-kneaded at a barrel temperature of 180 to 250 ° C, preferably 200 to 240 ° C is generally 150 to 270 ° C, preferably 170. The film is extruded from a T die or a hollow nozzle at a temperature of ˜240 ° C. Therefore, as long as a homogeneous composition in the above temperature range is finally obtained, the mixing and melting form of the vinylidene fluoride resin and the organic liquid are arbitrary. According to one preferred embodiment for obtaining such a composition, a biaxial kneading extruder is used, and the vinylidene fluoride resin (preferably comprising a mixture of a main resin and a crystal characteristic modifying resin) is The organic liquid is supplied from the upstream side of the extruder, supplied downstream, and made into a homogeneous mixture before being discharged through the extruder. This twin-screw extruder can be controlled independently by dividing it into a plurality of blocks along its longitudinal axis direction, and appropriate temperature adjustment is made according to the contents of the passing material at each site.

(cooling)
In the case of the thermally induced phase separation method, the melt-extruded hollow fiber membrane is then made of a liquid (preferably water) that is inert (that is, non-solvent and non-reactive) with respect to the vinylidene fluoride resin. It introduce | transduces in the cooling bath of temperature Tq low enough to raise | generate induced phase separation, Preferably it cools preferentially from the outer surface, and solidifies and forms a film. In order to form a flat film, cooling from one side by a chill roll is also used in addition to the cooling liquid shower. The lower the cooling temperature Tq, the smaller the hole diameter formed.

(Extraction)
The cooled and solidified film-like material is then introduced into the extract bath and subjected to extraction and removal of the organic liquid. The extraction liquid is not particularly limited as long as it does not dissolve the polyvinylidene fluoride resin and can dissolve the plasticizer and the like. For example, polar solvents having a boiling point of about 30 to 100 ° C. such as methanol and isopropyl alcohol are suitable for alcohols, and dichloromethane and 1,1,1-trichloroethane are suitable for halogenated solvents. It is efficient to extract the long hollow fiber membrane by winding it around a bobbin.

  Among them, the halogenated solvent has a high swellability with respect to vinylidene fluoride resin (a swelling rate measured by the following method is preferably 2 to 20% by weight, particularly preferably 5 to 10% by weight), and extraction of an organic liquid material. The effect is great. However, due to its swelling property, when the film-like product of vinylidene fluoride resin containing a halogenated solvent is transferred to the drying process as it is by extraction or the like, the formed pores tend to shrink. This tendency becomes more prominent as the membrane has a smaller pore size. Accordingly, a vinylidene fluoride resin porous membrane containing a halogenated solvent in the pores formed by halogenated solvent extraction is preferably compatible with the halogenated solvent and swells with respect to the vinylidene fluoride resin. It is preferable to dry after replacing the halogenated solvent by immersing it in a rinse solution made of a solvent having no property (preferably having a swelling ratio of less than 1% by weight measured by the following method). Specific examples of the solvent that is non-swelling and compatible with the halogenated solvent include isopropyl alcohol, ethanol, hexane, and the like. When a solvent that is compatible with water, such as isopropyl alcohol or ethanol, is used, the solvent is subsequently replaced with a non-swellable and non-flammable solvent such as water. Therefore, it is also preferable to perform drying or heat treatment.

<Swelling ratio measurement>
A vinylidene fluoride resin is heated and pressed at a temperature of 230 ° C. for 5 minutes, and then cooled and solidified by a cooling press at a temperature of 20 ° C. to produce a press sheet having a thickness of 0.5 mm. This press sheet is cut into 50 mm squares to form test pieces. After measuring the weight W1 of this test piece, it is immersed in a solvent at room temperature for 120 hours. Thereafter, the test piece is taken out, the solvent adhering to the surface is wiped off with a filter paper, and the weight W2 of the test piece is measured. The swelling rate (%) is measured by the following formula.

                  Swell rate (%) = (W2-W1) / W1 × 100.

(Stretching)
As an example, including the vinylidene fluoride resin porous membrane obtained as described above, preferably the porosity is 50% or more, more preferably 60% or more, the upper limit is 90% or less for various resin porous membranes, In general, stretching is performed for the purpose of increasing the porosity and pore diameter and improving the strength and elongation. In the present invention, prior to stretching, the resin is selectively wetted from the outer surface of the porous resin membrane to a certain depth. Stretching in a state (hereinafter referred to as “partial wet stretching”). This increases the porosity of the wet surface layer (the porosity of the portion from the surface to a depth of 5 μm is referred to as dense layer porosity A1). In general, a resin porous membrane used as a separation membrane does not have a uniform porosity distribution in the thickness direction, and a dense layer having a small pore diameter is formed on the surface layer, and this often dominates the separation performance. In addition, for example, a porous resin membrane formed by a thermally induced phase separation method has a strong tendency to obtain an inclined porous membrane in which one surface pore diameter in direct contact with the cooling medium is smaller than the other surface pore diameter. It is done. Partially wet stretching according to the present invention is extremely effective in increasing the dense layer porosity A1 and giving a good balance between good separation and liquid permeability.

  More specifically, prior to stretching, 5 μm or more from the outer surface of the porous membrane, preferably 7 μm or more, more preferably 10 μm or more, and ½ or less, preferably １ ／ or less, more preferably 1 / １ ／ of the film thickness. A depth of 4 or less is selectively wetted. If the depth of wetting is less than 5 μm, the increase in the dense layer porosity A1 is not sufficient, and if it exceeds 1/2, the drying of the wetting liquid becomes uneven when the dry heat is relaxed after stretching, and the heat treatment or the relaxation treatment May become uneven.

  Although the reason why the dense layer porosity A1 is improved by the partial wet stretching is not clear, the present inventors presume as follows. That is, it is thought that compressive stress acts in the film thickness direction when stretching in the longitudinal direction, but by wetting a certain depth from the outer surface, (a) heat transfer in a heated atmosphere is improved. In particular, the temperature gradient of the dense layer is relaxed and the compressive stress in the film thickness direction is reduced. (B) Since the liquid is filled in the voids, the compressive stress in the film thickness direction is increased by stretching. It is presumed that the hole is less likely to collapse even if it works.

  The operation of wetting only a predetermined depth from the outer surface of the resin porous membrane is preferably performed by bringing a partial wetting liquid adjusted in surface tension into contact with the outer surface of the resin porous membrane. As the partial wetting liquid, it is possible to use a solvent that wets the porous resin membrane or an aqueous solution thereof, but an aqueous surfactant solution (emulsion) because it is easy to wash and remove after stretching or no special removal operation is required. Including solutions).

  The surface tension of the partial wetting liquid is a porous membrane in the case of a hydrophobic resin porous membrane having a critical surface tension γc of less than 72 mN / m (mostly less than 60 mN / m) such as polyolefin resin or vinylidene fluoride resin. The surface tension of the surfactant aqueous solution is (γc-10) to (γc + 5) (mN / m), preferably (γc-8), depending on the critical surface tension γc (mN / m) (measurement method will be described later). ) To (γc + 3) (mN / m), and by controlling the liquid contact time from 5 seconds to 10000 seconds, only a predetermined depth from the surface of the porous resin membrane is wetted with the surfactant aqueous solution. I can do it.

  On the other hand, in the case of a hydrophilic resin porous membrane having a critical surface tension γc of 72 mN / m or more such as cellulose acetate-based resin, it is difficult to control the wet depth with the surface tension of the surfactant aqueous solution. In this case, by selecting a surfactant having an HLB (hydrophilic hydrophobic balance) of 3.5 to 10 as the surfactant to be added and controlling the liquid contact time from 5 seconds to 600 seconds, the resin porous membrane is obtained. Only a predetermined depth from the surface can be wetted with an aqueous surfactant solution. When HLB is selected in this manner, the emulsion particle size dispersed in the surfactant aqueous solution is close to the surface pore size of the membrane, and therefore the wet depth is considered to be controlled by the clogging effect of the emulsion particles.

  Such control of the wet depth by the clogging effect of the emulsion particles can also be used in the case of a hydrophobic resin. However, in the case of a hydrophobic resin, the HLB is preferably 8-20 in view of reducing the surface tension of the partial wetting liquid.

  In both cases of the hydrophobic resin and the hydrophilic resin, there is no particular restriction on the viscosity of the partial wetting liquid, but depending on the application method of the partial wetting liquid, the penetration rate can be appropriately adjusted by making the partial wetting liquid high in viscosity. It is possible to slow down or increase the penetration rate by lowering the viscosity. Also, the temperature of the partial wetting liquid is not particularly limited, but depending on the application method of the partial wetting liquid, the permeation rate can be lowered moderately by lowering the partial wetting liquid, or the permeation rate can be increased to a higher temperature. Can be made faster. Thus, the viscosity and temperature of the partial wetting liquid act in opposite directions, and can be complementarily controlled to adjust the penetration rate of the partial wetting liquid.

  For example, when it is desired to suppress the permeation rate, the temperature of the partial wetting liquid is preferably 0 to 25 ° C, more preferably 3 to 15 ° C, and still more preferably 5 to 10 ° C.

    As an example, in order to selectively wet a porous membrane made of vinylidene fluoride resin (PVDF) (the critical surface tension γc of a porous membrane made of PVDF homopolymer is 35 to 40 mN / m) from the outer surface to a certain depth. It is possible to apply a solvent that wets vinylidene fluoride resin such as methanol or ethanol or its aqueous solution to the outer surface of the porous membrane, but in order to wet it to a certain depth, surface tension Application of a partial wetting liquid having a viscosity of 25 to 45 mN / m (including the case of immersion) is preferred. If the surface tension is less than 25 mN / m, it may be difficult to selectively apply the partial wetting liquid to the outer surface because the penetration rate into the PVDF porous membrane is too fast. It may be difficult to apply the partial wetting liquid uniformly on the outer surface because it is repelled on the surface (insufficient wettability or permeability to the PVDF porous membrane). In particular, the use of an aqueous surfactant solution obtained by adding a surfactant to water (that is, an aqueous surfactant solution or an aqueous emulsion solution) is preferred as the partial wetting liquid. The type of the surfactant is not particularly limited. In the anionic surfactant, a carboxylate type such as an aliphatic monocarboxylate, a sulfonate type such as an alkylbenzene sulfonate, a sulfate ester type such as an alkyl sulfate, Phosphate ester type such as alkyl phosphate salt; amine salt type such as alkylamine salt for cationic surfactant, quaternary ammonium salt type such as alkyltrimethylammonium salt; glycerin fatty acid for nonionic surfactant Ester type such as ester, ether type such as polyoxyethylene alkylphenyl ether, ester ether type such as polyethylene glycol fatty acid ester; in amphoteric surfactant, carboxybetaine type such as N, N-dimethyl-N-alkylaminoacetic acid betaine 2-alkyl-1-hydroxyl Le - such as glycine type and the like, such as carboxymethyl imidazolinium betaine. In particular, polyglycerin fatty acid ester is preferably used as a partial wetting liquid that does not cause sanitary problems even if it finally remains in the porous membrane.

  The surfactant preferably has an HLB (hydrophilic / lipophilic balance) of 8 or more. When the HLB is less than 8, the surfactant is not finely dispersed in water, and as a result, uniform wettability improvement becomes difficult. Particularly preferably used surfactants include nonionic surfactants having an HLB of 8 to 20, and further 10 to 18, or ionic (anionic, cationic and amphoteric) surfactants. A surfactant is preferred.

  In many cases, application of the partial wetting liquid to the outer surface of the porous membrane is preferably performed by batch or continuous immersion of the porous membrane. This dipping process is a double-sided coating process for flat membranes and a single-sided coating process for hollow fiber membranes. The flat membrane batch dipping treatment is carried out by dipping the hollow fiber membranes bundled by bobbin winding or casserole winding by dipping the layers cut into appropriate sizes. In the case of batch processing, it is preferred to form relatively large emulsion particles using a surfactant having a relatively low HLB within the above range, more specifically 8-13. In the case of a flat membrane and a hollow fiber membrane, the continuous treatment is performed by continuously immersing a long porous membrane in the treatment liquid. When selectively applying only to one side of the flat film, spraying of the treatment liquid is also preferably used. In the case of continuous processing, a relatively small emulsion particle may be formed using a surfactant having a relatively high HLB within the above range, more specifically 8-20, more preferably 10-18. preferable.

  The partially porous resin porous membrane is stretched in an atmosphere (or medium) that is non-wetting with respect to the resin porous membrane. As the non-wetting atmosphere, a non-wetting liquid having a surface tension (JIS K6768) larger than the critical surface tension γc of the porous film by about 10 mN / m or almost all gases including air is used near room temperature. It is done. In the case of a hydrophobic resin porous membrane (many γc is less than 60 mN / m), preferably water or air is used, more preferably water. On the other hand, in the case of a hydrophilic resin porous membrane, water cannot be used because it is wettable, but almost all gases including air are preferably used.

  The stretching method itself may be carried out by a known method for each hollow fiber membrane and flat membrane of the resin. For example, the stretching of the vinylidene fluoride resin hollow fiber membrane is generally preferably performed as uniaxial stretching in the longitudinal direction of the hollow fiber membrane by a pair of rollers having different peripheral speeds. The stretching ratio is 1.5 times or more, preferably 1.7 times or more, and the upper limit is suitably 4 times or less. When the draw ratio is excessive, the tendency of the hollow fiber membrane to break becomes large. The stretching temperature is preferably 25 to 90 ° C, particularly 45 to 80 ° C. If the stretching temperature is too low, the stretching becomes non-uniform and the hollow fiber membrane is easily broken. On the other hand, if the stretching temperature is too high, it is difficult to increase the porosity even if the stretching ratio is increased. In the case of a flat membrane, uniaxial or sequential or simultaneous biaxial stretching is possible, and the stretching ratio is 1.5 to 20 times, preferably about 2.0 to 10 times as the area magnification. In order to improve the drawing operability, the crystallinity is increased by heat treatment at a temperature in the range of 80 to 160 ° C., preferably 100 to 140 ° C., for 1 second to 18000 seconds, preferably 3 seconds to 3600 seconds. Then, it is also preferable to apply the partial wet stretching of the present invention.

(Heat treatment)
The stretched porous membrane obtained as described above is preferably subjected to heat treatment under relaxation or constant length conditions to improve the dimensional stability of the stretched porous membrane. The heat treatment of the stretched porous membrane made of a hydrophobic resin is carried out in a non-wetting atmosphere with respect to the resin porous membrane, for example, in the range of 50 ° C. to (melting point Tm 1-5 ° C.) for the crystalline resin, That is, with respect to the resin whose melting point Tm1 is not detected, it is preferable to perform one-step or two-step treatment in the range of 50 ° C. to (glass transition temperature−5 ° C.). For example, a non-wetting atmosphere with respect to a hydrophobic resin such as a vinylidene fluoride resin is a non-wetting liquid having a surface tension (JIS K6768) larger than the wetting tension of the hydrophobic resin near room temperature, typically Almost all gases including water or air are used.

  As an example, the relaxation conditions for the vinylidene fluoride resin will be described. The relaxation treatment of the uniaxially stretched porous membrane like the hollow fiber is arranged between the upstream roller and the downstream roller where the peripheral speed is gradually reduced. It is obtained by passing the previously obtained stretched porous membrane through a non-wetting, preferably heated atmosphere. The relaxation rate determined by (1− (downstream roller peripheral speed / upstream roller peripheral speed)) × 100 (%) is preferably in the range of 0% (constant-length heat treatment) to 50%, particularly 1 to 20 % Relaxation heat treatment is preferable. A relaxation rate exceeding 20% is not preferable because it depends on the stretching ratio in the previous step, but is difficult to realize or even if realized, the stretching effect is saturated or decreases. In the case of a flat membrane, the uniaxial or biaxial relaxation rate is suitably about 1 to 30% as the area relaxation rate. When the two-stage treatment is performed, the first-stage constant length or relaxation heat treatment temperature is preferably 50 to 100 ° C., particularly preferably 80 to 100 ° C. The treatment time may be short or long as long as the desired heat setting effect and relaxation rate are obtained. Generally, it is about 5 seconds to 1 minute, but it is not necessary to be within this range. The post-stage constant length or relaxation heat treatment temperature is 80 to 170 ° C., particularly 120 to 160 ° C., and it is preferable that a desired relaxation rate is obtained in total with the first stage.

(Stretched porous membrane)
Although the porous film obtained through the steps of stretching (and heat treatment) varies depending on the resin, according to a preferred embodiment, the average pore diameter on the small pore diameter side surface and the half-dry method average pore diameter are each preferably 0.5 μm. Hereinafter, the porosity of the dense layer adjacent to the surface on the small pore diameter side as well as the entire membrane is maintained within the range where 0.3 μm or less, more preferably 0.2 μm or less (the lower limit is 0.01 μm) is maintained. The feature of maintaining high liquid permeability is given.

  Hereinafter, the method for applying the partially wet stretching step of the present invention to the vinylidene fluoride resin hollow fiber porous membrane by the heat-induced phase separation method will be described in more detail with reference to the production method of the stretched resin porous membrane of the present invention. As will be described, it will be readily understood by those skilled in the art that the method of the present invention can be applied to the production of more various flat resin membranes or hollow fiber porous membranes within the scope of the present invention. The characteristic values described in this specification, including the following description, are based on measured values by the following method.

(Crystal melting point Tm1, Tm2 and crystallization temperature Tc, Tc ′)
Using a differential scanning calorimeter “DSC7” manufactured by PerkinElmer Co., Ltd., 10 mg of sample resin was set in a measurement cell, and the temperature was increased from 30 ° C. to 250 ° C. at a rate of 10 ° C./min in a nitrogen gas atmosphere. The temperature was then maintained at 250 ° C. for 1 minute, and then the temperature was decreased from 250 ° C. to 30 ° C. at a rate of 10 ° C./min to obtain a DSC curve. In the DSC curve, the endothermic peak speed in the temperature rising process was the melting point Tm1 (° C.), and the exothermic peak temperature in the temperature lowering process was the crystallization temperature Tc (° C.). Subsequently, after maintaining at a temperature of 30 ° C. for 1 minute, the temperature was raised again from 30 ° C. to 250 ° C. at a rate of 10 ° C./min, and the DSC curve was measured. The endothermic peak temperature in this reheated DSC curve was the original resin melting point Tm2 (° C.) that defines the crystal characteristics of the vinylidene fluoride resin of the present invention.

  The crystallization temperature Tc ′ (° C.) of the mixture of vinylidene fluoride resin as a film raw material and a plasticizer is the first intermediate molded body that is melt-kneaded by an extruder and then extruded from a nozzle and cooled and solidified. The exothermic peak temperature detected in the temperature lowering process is obtained by obtaining a DSC curve through the same heating and cooling cycle as described above using 10 mg of the sample.

(Compatibility)
The compatibility of an organic liquid material (hereinafter simply referred to as “plasticizer” in this section) composed of a polyester plasticizer and a monomeric ester plasticizer with respect to a vinylidene fluoride resin was determined by the following method:
A slurry-like mixture is obtained by mixing 23.73 g of vinylidene fluoride resin and 46.27 g of a plasticizer at room temperature. Next, the barrel of “Lab Plast Mill” (mixer type: “R-60”) manufactured by Toyo Seiki Co., Ltd. is 10 ° C. or higher (for example, about 17 to 37 ° C. higher) than the melting point of the vinylidene fluoride resin. The temperature is adjusted, and the slurry mixture is charged and preheated for 3 minutes, and then melt-kneaded at a mixer rotation speed of 50 rpm. When a melted and kneaded product that is clear (that is, transparent to the extent that there is no visible turbidity) is obtained within 10 minutes after the start of kneading, the plasticizer is based on vinylidene fluoride resin. Determined to be compatible. In addition, when the viscosity of the melt-kneaded material is high, it may appear cloudy due to entrapment of bubbles, and in that case, it is determined by deaeration appropriately by a method such as hot pressing. Once it has cooled and solidified, it is heated again to be in a molten state, and then it is determined whether or not it is clarified.

(Weight average molecular weight (Mw))
Using a GPC device “GPC-900” manufactured by JASCO Corporation, using “Shodex KD-806M” manufactured by Showa Denko Co., Ltd. as a column, “Shodex KD-G” used as a precolumn, NMP as a solvent, temperature 40 ° C., flow rate The molecular weight was measured in terms of polystyrene by gel permeation chromatography (GPC) at 10 mL / min.

(All layer porosity A2)
The apparent volume V (cm ³ ) of the porous membrane including the flat membrane and the hollow fiber membrane was calculated, and the weight W (g) of the porous membrane was measured to determine the total-layer porosity A2 from the following formula:
[Equation 1]
Total layer porosity A2 (%) = (1−W / (V × ρ)) × 100
ρ: Specific gravity of PVDF (= 1.78 g / cm ³ ).

(Average pore diameter)
Based on ASTM F316-86 and ASTM E1294-89, average pore diameter Pm (μm) was measured by a half dry method using “Palm Porometer CFP-200AEX” manufactured by Porous Materials, Inc. Perfluoropolyester (trade name “Galwick”) was used as a test solution.

(Maximum hole diameter)
Based on ASTM F316-86 and ASTM E1294-89, the maximum pore size Pmax (μm) was measured by a bubble point method using “Palm Porometer CFP-200AEX” manufactured by Porous Materials, Inc. Perfluoropolyester (trade name “Galwick”) was used as a test solution.

(Treatment water side surface hole diameter P1 and permeate water side surface hole diameter P2)
For a flat membrane or hollow fiber-like porous membrane sample, the average pore diameter P1 of the treated water side surface (outer surface in hollow fiber) and the average pore diameter P2 of the permeated water side surface (inner surface in hollow fiber) were determined by SEM method. Measured (SEM average pore diameter). Hereinafter, the measurement method will be described using a hollow fiber porous membrane sample as an example. SEM photography is performed on the outer surface and inner surface of the hollow fiber membrane sample at an observation magnification of 15,000 times, respectively. Next, for each SEM photograph, the hole diameter is measured for everything that can be recognized as a hole. The hole diameter is obtained by measuring the long and short diameters of each hole and determining the hole diameter = (long diameter + short diameter) / 2. The arithmetic average of the measured pore diameters is obtained and set as the outer surface average pore diameter P1 and the inner surface average pore diameter P2. In addition, when there are too many holes observed in the photograph, the photograph image may be divided into four equal parts, and the above-mentioned hole diameter measurement may be performed for one area (¼ screen). . When the outer surface of the hollow fiber membrane of the present invention is measured on a ¼ screen, the number of measurement holes is approximately 200 to 300.

(Dense layer porosity)
For a flat membrane or hollow fiber-like porous membrane sample, the porosity A1 (%) of the portion having a thickness of 5 μm in contact with the surface on the small pore diameter side (the outer surface for the hollow fiber membrane) (hereinafter referred to as “dense layer porosity A1”) Is measured by the impregnation method. Hereinafter, the measurement method will be described by taking a hollow fiber porous membrane sample as an example. First, a hollow fiber porous membrane sample is cut into a length L = about 300 mm, both ends of the hollow portion are sealed with thermocompression bonding or an adhesive, and the weight W0 (g) is measured. Next, a hollow fiber membrane sample sealed at both ends was prepared by adding 0.05% by weight of a dye ("Cation Red" manufactured by Kiwa Chemical Industry Co., Ltd.) and "MO-" produced by a fatty acid glycerin ester (Sakamoto Pharmaceutical Co., Ltd.). 7S "; HLB value = 12.9) After immersion in a test solution made of glycerin dissolved in approximately 1.0% by weight (" Purified Glycerin D "manufactured by Lion Corporation), the surface test solution was taken out and wiped off. The weight W (mg) is measured again. Next, the sample after the measurement is cut with a razor, and the thickness t (μm) of the portion impregnated with the test solution (= stained portion) is measured using an optical microscope (“VQ-Z50” manufactured by KEYENS). The thickness t is adjusted to t = 5 ± 1 (μm) by adjusting the immersion time in the test solution and the concentration of the aliphatic glycerin ester in the test solution. From the thickness L (mm) and the impregnation thickness t (μm), the volume V (ml) of the portion of the sample impregnated with the test liquid is calculated by the following formula:
V = π × ((OD / 2) ² − (OD / 2−t / 1000) ² ) × L / 1000
From the difference between the weight W0 (g) of the sample before immersion and the weight W (g) of the sample after immersion, the volume VL (ml) of the test solution impregnated is calculated by the following formula:
VL = (W−W0) / (ρs × 1000)
Here, ρs is the specific gravity of the test solution and is 1.261 (g / ml).

  The dense layer porosity A1 (%) is calculated by the following formula.

       A1 = VL / V × 100.

(Water permeability)
Sample length L (see FIG. 1) = 200 mm of sample hollow fiber porous membrane was immersed in ethanol for 15 minutes, then immersed in pure water for 15 minutes, wetted, and then measured at a water temperature of 25 ° C. and a differential pressure of 100 kPa per day. The water permeation amount (m ³ / day) was divided by the membrane area (m ² ) of the hollow fiber porous membrane (= calculated as outer diameter × π × test length L). The measured value is expressed as F (100 kPa, L = 200 mm), and the unit is represented by m / day (= m ³ / m ² · day).

(surface tension)
The surface tension of the wet treatment liquid at a temperature of 25 ° C. was measured by a ring method according to JIS-K3362 using a Dunui surface tension tester.

(Critical surface tension)
Water and ethanol were mixed at different ratios to prepare aqueous solutions with different surface tensions. The relationship between the ethanol concentration and the surface tension was referred to the description in the Chemical Engineering Handbook (Maruzen Co., Ltd., revised fifth edition). In the measurement of the water permeability, pure water permeability F ′ (m / day) (= m ³ / m ² / day) was measured using the above aqueous solution instead of wetting the porous membrane with ethanol. The maximum surface tension at which the ratio F ′ / F to the pure water permeation amount F measured by wetting with ethanol alone is 0.9 or more is defined as the critical surface tension of the porous membrane. Incidentally, the critical surface tension γc of the vinylidene fluoride resin hollow fiber porous membrane formed in Examples 1 to 5 described later was measured to be 38 mN / m.

(Tensile test)
Using a tensile tester (“RTM-100” manufactured by Toyo Baldwin Co., Ltd.), the measurement was performed under the conditions of an initial sample length of 100 mm and a crosshead speed of 200 mm / min in an atmosphere at a temperature of 23 ° C. and a relative humidity of 50%.

Example 1
Polyvinylidene fluoride for matrix (PVDF-I) (powder) having a weight average molecular weight (Mw) of 4.9 × 10 ⁵ and polyvinylidene fluoride for modifying crystal properties (PVDF-II) having a Mw of 9.7 × 10 ⁵ ) (Powder) at a ratio of 75 wt% and 25 wt%, respectively, was mixed using a Henschel mixer to obtain a PVDF mixture having an Mw of 6.1 × 10 ⁵ .

  As an organic liquid, a polyester plasticizer (polyester of adipic acid and 1,2-butanediol whose ends are sealed with isononyl alcohol; “D623N” manufactured by J Plus Co., Ltd., number average molecular weight of about 1800, JIS K7117- 2 (cone-flat-plate rotational viscometer) measured viscosity at 25 ° C. 3000 mPa · s, specific gravity 1.090 g / ml), and diisononyl adipate which is a monomeric ester plasticizer (“DINA” manufactured by J. Plus) And a plasticizer mixture obtained by stirring and mixing at a normal temperature of 88% / 12% by weight.

  Using a co-rotating meshing twin screw extruder (“TEM-26SS” manufactured by Toshiba Machine Co., Ltd., screw diameter 26 mm, L / D = 60), the mixture A is supplied from the powder supply unit, and the barrel temperature is 220 ° C. Then, the plasticizer is mixed at a ratio of mixture A / plasticizer = 27.9 wt% / 72.1 wt% from the liquid supply section provided downstream from the powder supply section of the extruder cylinder. The mixture was fed and kneaded at a barrel temperature of 220 ° C., and the mixture was extruded into a hollow fiber shape from a nozzle (190 ° C.) having a circular slit having an outer diameter of 6 mm and an inner diameter of 4 mm. At this time, the inner diameter was adjusted by injecting air into the hollow portion of the hollow fiber from the vent provided in the center of the nozzle.

  The extruded mixture is kept in a molten state, maintained at a temperature of 45 ° C., and has a water surface at a position 280 mm away from the nozzle (ie, the air gap is 280 mm) and is cooled and solidified in a water cooling bath at a temperature Tq = 45 ° C. (Retention time in the cooling bath: about 6 seconds) After taking up at a take-up speed of 3.8 m / min, this is wound around a bobbin with a length of 500 m, and the first having an outer diameter of 1.80 mm and an inner diameter of 1.20 mm. An intermediate molded body was obtained.

  Next, the plasticizer was extracted by immersing the first intermediate molded body in dichloromethane at room temperature for 30 minutes. At this time, the extraction was performed while rotating the bobbin so that the dichloromethane was evenly distributed over the yarn. Next, the operation of replacing the dichloromethane with a new one and extracting again under the same conditions was repeated, and extraction was performed three times in total.

  Next, the first intermediate molded body containing dichloromethane is immersed in isopropyl alcohol (IPA) at room temperature for 30 minutes without substantially drying (that is, no whitening is visually recognized in the first intermediate molded body). The dichloromethane impregnated in the first intermediate molded body was replaced with IPA. At this time, the replacement was performed while rotating the bobbin so that the IPA was evenly distributed over the yarn. Subsequently, the operation of replacing the IPA with a new one and replacing the same under the same conditions was repeated, and the replacement was performed twice in total.

  Next, it was air-dried at room temperature for 24 hours to remove IPA, followed by heating in an oven at a temperature of 120 ° C. for 1 hour to remove IPA and heat treatment to obtain a second intermediate molded body. At this time, the bobbin diameter was freely contracted, and drying and heat treatment were performed without restricting the contraction of the yarn.

Next, in a state where the second intermediate molded body is wound around a bobbin, a polyglycerin fatty acid ester (“SY Glyster ML-310” manufactured by Sakamoto Pharmaceutical Co., Ltd., HLB = 1) is used as a surfactant.
0.3) was dissolved in pure water at a concentration of 0.05% by weight (surface tension = 32.4 mN / m) and immersed for 30 minutes at room temperature to perform partial wetting.

  Further, while the bobbin is immersed in the emulsion aqueous solution, the second intermediate molded body is pulled out while rotating the bobbin, the first roll speed is set to 20.0 m / min, and the second roll is passed through a 60 ° C. water bath. The film was stretched 1.75 times in the longitudinal direction by setting the speed to 35.0 m / min. Next, it is passed through a hot water bath controlled at a temperature of 90 ° C., the first stage relaxation rate is relaxed at 8%, and further passed through a dry heat tank controlled at a spatial temperature of 140 ° C., and the second stage relaxation rate is 1.5%. And relaxed. This was wound up to obtain a vinylidene fluoride resin hollow fiber porous membrane.

(Example 2)
A vinylidene fluoride resin porous membrane was obtained in the same manner as in Example 1 except that the cooling water bath temperature Tq after the melt extrusion was changed to 30 ° C .; and the draw ratio was changed to 1.85 times.

(Example 3)
Polyester plasticizer (polyester of dibasic acid and glycol whose ends are sealed with benzoic acid; “W-83” manufactured by DIC Corporation, number average molecular weight of about 500, JIS K7117-2 (cone- A viscosity measured at 25 ° C. by a flat plate type viscometer (750 mPa · s, specific gravity 1.155 g / ml); vinylidene fluoride resin / plasticizer = 26.9 wt% / 73.1 wt% A vinylidene fluoride resin porous membrane was obtained in the same manner as in Example 1 except that the cooling water bath temperature Tq after melt extrusion was changed to 50 ° C.

Example 4
As an organic liquid material, an alkylene glycol dibenzoate which is a monomeric ester plasticizer (“PB-10” manufactured by DIC Corporation, number average molecular weight of about 300, at 25 ° C. by JIS K7117-2 (cone-plate type rotary viscometer)). Measured viscosity of 81 mPa · s, specific gravity 1.147 g / ml); vinylidene fluoride resin / plasticizer = 26.9 wt% / 73.1 wt% supplied; after melt extrusion A vinylidene fluoride resin porous membrane was obtained in the same manner as in Example 1 except that the cooling water bath temperature Tq was changed to 60 ° C. and the second-stage relaxation rate was changed to 1.5%.

(Example 5)
Essentially, an unstretched vinylidene fluoride resin porous membrane was obtained according to the method disclosed in Patent Document 4, and then the unstretched yarn was partially wetted and then stretched.

That is, hydrophobic silica ("Aerosil R-972" manufactured by Nippon Aerosil Co., Ltd., primary particle average diameter 16 nanometer, specific surface area 110 m < ² > / g) 14.8 vol%, dioctyl phthalate (DOP) 48.5 vol% %, Dibutyl phthalate (DBP) 4.4% by volume is mixed with a Henschel mixer, and 32.3% by volume of polyvinylidene fluoride (powder) having a weight average molecular weight (Mw) of 2.4 × 10 ⁵ is mixed therewith. Added and mixed again with a Henschel mixer.

  This mixture was supplied to a co-rotating meshing twin screw extruder (“TEM-26SS” manufactured by Toshiba Machine Co., Ltd., screw diameter 26 mm, L / D = 60), kneaded at a barrel temperature of 240 ° C., and the mixture was removed. A hollow fiber was extruded from a nozzle (temperature 240 ° C.) having a circular slit with a diameter of 6 mm and an inner diameter of 4 mm. At this time, the inner diameter was adjusted by injecting air into the hollow portion of the hollow fiber from the vent provided in the center of the nozzle.

  The extruded mixture is kept in a molten state, maintained at a temperature of 70 ° C., and has a water surface at a position 140 mm away from the nozzle (ie, the air gap is 140 mm). (Residence time in the cooling bath: about 9 seconds) was taken at a take-up speed of 2.5 m / min to obtain a first intermediate molded body having an outer diameter of 2.87 mm and an inner diameter of 1.90 mm.

  Next, the plasticizer was extracted by immersing the first intermediate molded body in dichloromethane at room temperature for 30 minutes. Next, the operation of replacing the dichloromethane with a new one and extracting again under the same conditions was repeated, and extraction was performed four times in total. Next, it was dried in a vacuum dryer at a temperature of 30 ° C. for 24 hours to remove dichloromethane.

  Next, it was immersed in 50% ethanol aqueous solution for 30 minutes, and further immersed in pure water for 30 minutes to wet the hollow fiber. Further, after removing the hydrophobic silica by immersing in a 20% aqueous sodium hydroxide solution at 70 ° C. for 1 hour, it is washed with water to remove the sodium hydroxide, and then dried in a vacuum dryer at a temperature of 30 ° C. for 24 hours. A molded body was obtained. In addition, during a series of operations from extraction to drying, both ends of the hollow fiber were not contracted and were freely contracted.

  Next, after sealing both ends of the second intermediate molded body, a polyglycerol fatty acid ester (“SY Glyster ML-310”, HLB = 10.3, manufactured by Sakamoto Yakuhin Kogyo Co., Ltd.) is used as a surfactant. Partial wetting was performed by immersing in an aqueous emulsion (surface tension = 32.4 mN / m) dissolved in pure water at 0.05% by weight at room temperature for 60 minutes. Next, after stretching by hand 1.5 times in the longitudinal direction in a room temperature atmosphere, heat treatment was carried out for 5 minutes in a hot air oven at a temperature of 140 ° C. with the length fixed, thereby forming a vinylidene fluoride resin hollow fiber porous membrane. Obtained.

(Comparative Example 1)
A vinylidene fluoride resin hollow fiber porous membrane was obtained in the same manner as in Example 1 except that partial wetting was not performed prior to stretching.

(Comparative Example 2)
A vinylidene fluoride resin hollow fiber porous membrane was obtained in the same manner as in Example 2 except that partial wetting was not performed prior to stretching.

(Comparative Example 3)
Vinylidene fluoride resin in the same manner as in Example 2 except that an aqueous solution (surface tension = 28.9 mN / m) of sodium alkyl ether sulfate dissolved in pure water at a concentration of 0.05% by weight was used as the partial wetting liquid. A hollow fiber porous membrane was obtained.

(Comparative Example 4)
A vinylidene fluoride resin hollow fiber porous membrane was obtained in the same manner as in Example 3 except that partial wetting was not performed prior to stretching.

(Comparative Example 5)
A vinylidene fluoride resin hollow fiber porous membrane was obtained in the same manner as in Example 4 except that partial wetting was not performed prior to stretching.

(Comparative Example 6)
A vinylidene fluoride resin hollow fiber porous membrane was obtained in the same manner as in Example 5 except that partial wetting was not performed prior to stretching.

  The outline of the production conditions of the above Examples and Comparative Examples and the physical properties of the obtained polyvinylidene fluoride hollow fiber porous membrane are summarized in Tables 1 and 2 below.

  As will be understood by comparing the results of Examples and Comparative Examples shown in Tables 1 and 2 above, according to the method of the present invention, the surface of the porous resin membrane once formed is selectively partially wetted. By stretching the film, the porosity in the vicinity of the surface layer during stretching is prevented, and as a result, the porosity A1 of the dense layer in the vicinity of the surface layer that controls the separation performance is increased and the permeability of the entire membrane is increased. A porous resin membrane having high liquidity can be obtained. In particular, this result shows that the small-pore-diameter side surface pore diameter P1 is 0.2 μm or less compared to the case where the small-pore-diameter-side surface pore diameter P1 that governs the separation performance is relatively large as about 1 μm (Example 5, Comparative Example 6). In the case where it is small (Examples 1 to 4, Comparative Examples 1 to 5), this is particularly noticeable.

Claims (14)

  A method for producing a stretched resin porous membrane, comprising stretching a resin porous membrane in a state of being selectively wetted with a wetting liquid to a depth of 5 μm or more and 1/2 or less of the film thickness from the outer surface thereof .   The manufacturing method according to claim 1, wherein stretching is performed in a state of being selectively wetted from the outer surface to a depth of 7 μm or more and ½ or less of the film thickness.   The manufacturing method of Claim 1 or 2 which extends | stretches the resin porous membrane whose porosity is 50% or more.   The production method according to any one of claims 1 to 3, wherein the resin porous membrane is an asymmetric structure membrane having different pore diameters on two main surfaces, and wets only the surface having the smaller pore diameter.   The manufacturing method in any one of Claims 1-4 whose draw ratio is 1.5 times or more. The manufacturing method in any one of Claims 1-5 in which a resin porous film consists of hydrophobic resin. The manufacturing method in any one of Claims 1-5 in which a resin porous film consists of vinylidene fluoride resin. The production method according to claim 6 or 7, wherein the wetting liquid is an aqueous solution. The manufacturing method of Claim 8 whose wetting liquid is surfactant aqueous solution. The manufacturing method of Claim 8 whose wetting liquid is the aqueous solution of polyglyceryl fatty acid ester.   The manufacturing method according to any one of claims 1 to 0, wherein the pore diameter of the resin porous membrane after stretching has a surface pore diameter of 0.5 µm or less.   The method according to any one of claims 1 to 11, wherein the stretched resin porous membrane has a half-dry method average pore size of 0.5 µm or less.   The manufacturing method in any one of Claims 1-12 whose extending | stretching temperature is 25-90 degreeC.   The manufacturing method in any one of Claims 1-13 including the relaxation process in the liquid or gas which does not wet the resin porous membrane after extending | stretching.

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